Scenes:  Implement more item positioning features

There are a ridiculous number of features related to scaling and
positioning due to requests by a number of people who complained that
they hated the way that OBS1 would always resize their sources when the
source's base size changed.  There were also people who wanted more
control for how the resizing was handled, or the ability to completely
prevent resizing entirely if desired.  So I made it so that you can
optionally use a 'bounds' system, which allows you to specify different
styles of controlling resizing.

If disabled, the source will always automatically resize and only the
base scale is applied.  If enabled, you have a variety of different ways
to limit/control how it can resize within the bounds, or make it so it
can't resize at all.  You can also control alignment within that
bounding box, so you can make it so that a source always aligns to a
side or corner of the box.

I also added an alignment value which changes how the source is oriented
relative to the position of the scene item.  For example, setting
bottom-right alignment will make it so that the position of the item is
the bottom right corner of the source.  When the source resizies, it
will resize leftward and upward in that case, which solves the problem
of how a source resizes relative to a desired position.
UI:  Add scene editing

So, scene editing was interesting (and by interesting I mean
excruciating).  I almost implemented 'manipulator' visuals (ala 3dsmax
for example), and used 3 modes for controlling position/rotation/size,
but in a 2D editing, it felt clunky, so I defaulted back to simply
click-and-drag for movement, and then took a similar though slightly
different looking approach for handling scaling and reszing.

I also added a number of menu item helpers related to positioning,
scaling, rotating, flipping, and resetting the transform back to
default.

There is also a new 'transform' dialog (accessible via menu) which will
allow you to manually edit every single transform variable of a scene
item directly if desired.

If a scene item does not have bounds active, pulling on the sides of a
source will cause it to resize it via base scale rather than by the
bounding box system (if the source resizes that scale will apply).  If
bounds are active, it will modify the bounding box only instead.

How a source scales when a bounding box is active depends on the type of
bounds being used.  You can set it to scale to the inner bounds, the
outer bounds, scale to bounds width only, scale to bounds height only,
and a setting to stretch to bounds (which forces a source to always draw
at the bounding box size rather than be affected by its internal size).
You can also set it to be used as a 'maximum' size, so that the source
doesn't necessarily get scaled unless it extends beyond the bounds.

Like in OBS1, objects will snap to the edges unless the control key is
pressed.  However, this will now happen even if the object is rotated or
oriented in any strange way.  Snapping will also occur when stretching
or changing the bounding box size.
UI:  Add padding to scene edges for editing

Add a 10 pixel padding to the sides and remove viewport cutting to
ensure that the editing rectangles are visible even when in the upper
corners.

Also, add a black background for the actual 'scene' in the preview
window so that the scene boundries are actually visible in relation to
the rest of the preview space.
Merge pull request #114 from jp9000/scene-editing

Scene editing