Grease Pencil - Storyboarding Features (merge from GPencil_EditStrokes branch)

This merge-commit brings in a number of new features and workflow/UI improvements for
working with Grease Pencil. While these were originally targetted at improving
the workflow for creating 3D storyboards in Blender using the Grease Pencil,
many of these changes should also prove useful in other workflows too.

The main highlights here are:
1) It is now possible to edit Grease Pencil strokes
   - Use D Tab, or toggle the "Enable Editing" toggles in the Toolbar/Properties regions
     to enter "Stroke Edit Mode". In this mode, many common editing tools will
     operate on Grease Pencil stroke points instead.
   - Tools implemented include Select, Select All/Border/Circle/Linked/More/Less,
     Grab, Rotate, Scale, Bend, Shear, To Sphere, Mirror, Duplicate, Delete.
   - Proportional Editing works when using the transform tools

2) Grease Pencil stroke settings can now be animated
   NOTE: Currently drivers don't work, but if time allows, this may still be
         added before the release.

3) Strokes can be drawn with "filled" interiors, using a separate set of
   colour/opacity settings to the ones used for the lines themselves.

   This makes use of OpenGL filled polys, which has the limitation of only
   being able to fill convex shapes. Some artifacts may be visible on concave
   shapes (e.g. pacman's mouth will be overdrawn)

4) "Volumetric Strokes" - An alternative drawing technique for stroke drawing
   has been added which draws strokes as a series of screen-aligned discs.

   While this was originally a partial experimental technique at getting better
   quality 3D lines, the effects possible using this technique were interesting
   enough to warrant making this a dedicated feature. Best results when partial
   opacity and large stroke widths are used.

5) Improved Onion Skinning Support
   - Different colours can be selected for the before/after ghosts. To do so,
     enable the "colour wheel" toggle beside the Onion Skinning toggle, and set
     the colours accordingly.
   - Different numbers of ghosts can be shown before/after the current frame

6) Grease Pencil datablocks are now attached to the scene by default instead of
   the active object.
   - For a long time, the object-attachment has proved to be quite problematic
     for users to keep track of. Now that this is done at scene level, it is
     easier for most users to use.
   - An exception for old files (and for any addons which may benefit from object
     attachment instead), is that if the active object has a Grease Pencil datablock,
     that will be used instead.
   - It is not currently possible to choose object-attachment from the UI, but
     it is simple to do this from the console instead, by doing:
     context.active_object.grease_pencil = bpy.data.grease_pencil["blah"]

7) Various UI Cleanups
   - The layers UI has been cleaned up to use a list instead of the nested-panels
     design. Apart from saving space, this is also much nicer to look at now.

   - The UI code is now all defined in Python. To support this, it has been necessary
     to add some new context properties to make it easier to access these settings.
     e.g. "gpencil_data" for the datablock
          "active_gpencil_layer" and "active_gpencil_frame" for active data,
          "editable_gpencil_strokes" for the strokes that can be edited

   - The "stroke placement/alignment" settings (previously "Drawing Settings" at the
     bottom of the Grease Pencil panel in the Properties Region) is now located in
     the toolbar. These were more toolsettings than properties for how GPencil got drawn.

   - "Use Sketching Sessions" has been renamed "Continuous Drawing", as per a
     suggestion for an earlier discussion on developer.blender.org

   - By default, the painting operator will wait for a mouse button to be pressed
     before it starts creating the stroke. This is to make it easier to include
     this operator in various toolbars/menus/etc.   To get it immediately starting
     (as when you hold down DKEy to draw), set "wait_for_input" to False.

   - GPencil Layers can be rearranged in the "Grease Pencil" mode of the Action Editor

   - Toolbar panels have been added to all the other editors which support these.

8) Pie menus for quick-access to tools
   A set of experimental pie menus has been included for quick access to many
   tools and settings. It is not necessary to use these to get things done,
   but they have been designed to help make certain common tasks easier.

   - Ctrl-D = The main pie menu. Reveals tools in a context sensitive and
              spatially stable manner.
   - D Q    = "Quick Settings" pie. This allows quick access to the active
              layer's settings. Notably, colours, thickness, and turning
              onion skinning on/off.
Removing last vestigial remains of old C-based UI for Grease Pencil

I'd kept the code around in the codebase until after the merge back to master
to avoid having too many conflicts if things changed there (or in case we
needed to roll back). Now, it's safe to jettison this!
Fix for wrong RNA type causing asserts to fail in debug mode
Cleanup: tag unused args as such.
Typo fix for comment
Minor tweaks to quick palettes to avoid conflicts

For now, let's move the GPencil pies to avoid conflicts with dyntopo and other things
* DQ = Main pie (previously Ctrl-D; DD was too unpredictable)
* DW = Settings pie
Fix T42760: Drag-to-add feature fails in a case.

We have to unset 'name' when we set 'filepath', and vice-versa.
More fixes related to D&D:

* Same issue as T42760 was also reproducible in some cases in 3DView.
* You got an error message about missing RNA prop in some drop-called operators.
* You could not drop a movie file in nodes, (some cases of) 3DView, nor ImageEditor.
Cleanup: warnings & space
Fixed T35128. Progress bar sync on Windows.

Apply for all windows instead of active, otherwise progress doesn't update.
Fix T42426. Add support for UK "`" key.

Because key == OEM_8, there no clear conversion for different keyboard layouts.
Also, we must map key to GhostKey for shortcuts.
Bugfix T42763: In GPencil Edit Strokes: Proportional edit mode hidden when there is no active object in scene
Re-enabling keymappings for straight line and poly drawing

These are no longer blocked by the pie hotkeys, so they shouldn't be a problem
Improved Collada Armature Import

Collada does not support Bone length. It has only Joints.
This patch tries to improve the necessary "guesswork" for
importing Rigs.

Differential Revision: https://developer.blender.org/D920
Fix: Changing Grease Pencil datablocks updates the view
Fix: Forgot to set fill colour when drawing 2D filled strokes
Fix: Volumetric strokes now draw correctly in the Image Editor

This needed a correction factor (currently hardcoded to be 1 / 1000)
as it seems that the image editor uses 1 unit as its MAXIMUM dimension
whereas everything else uses 1 unit = 1 pixel.
Fix: Strokes in image editor can finally be drawn using "fancy" 2D stroke tesslation

Inspired by the previous commit, I've finally found a way to fix a long standing
limitation/bug which meant that Grease Pencil strokes in the Image Editor could
not be drawn using the fancy stroke tesselation code, and were instead done using
the plain old OpenGL strokes instead.
UI: auto-run error could push buttons off the visible header
CMake: fix for MSVC, installing Python