Add a seperate camera for play mode

Play mode now has a seperate entity to represent the camera. This is
automatically added to the scene if it doesn't already exist after
loading a file. The camera is not internal so it can be animated,
serialized and saved. The RigCameraView now uses this camera instead
of its view camera when in play mode. The entity has a handle entity
so that it can be selected and will be visible in the scene. However
it's currently quite difficult to find it because it is usually
positioned far away from the scene.
Reinstate the Ctrl+C key to select the camera

The Ctrl+C key used to select the view camera so that its properties
could be manipulated. This was removed since the switch to a separate
camera view in bf635357 made this difficult. This patch brings the
keybinding back but makes it select the global play camera instead of
the view camera. This is useful because it's not always easy to find
the handle for the play camera if it is far away from the scene.
camera-view: Add a button to move selected entity to camera position

Pressing Ctrl+J now moves the selected entity to the position of the
view camera and also copies its rotation. This will later be useful to
move the entity representing the camera for play mode to match the
view camera.
Add api for chaning scrollbar color + lighter default
Adds new RutIconButton widget

This adds a new button widget that lets you specify specific icons to
use for the different states; normal, hover, active and disabled. A
label can also be specified and you can specify whether the label should
be above, below or to the side of the icon.
Use icons for connect buttons
bit: queue redraw when setting child
for simple sizables return natural size of 0

This changes rut_simple_sizable_get_preferred_width/height to return 0
for the natural size instead of returning the current size.
shrink the light model
Add support for resetting the view by pressing 0
Hide and show light when entering/leaving play mode
center the labels for the asset selectors
add missing asset selector icons
add missing connect icon asset
Add video playback and pointalism effect

- Add a video texture element to RutMaterial
- Look for video assets
- Custom CoglPipeline and drawing for plain videos
  and pointalism effect.
Create asset selector buttons
fix: switch to connect button icon
Add more asset selector icons
Don't use a split view for the assets panel
simplify split view, not allowing interactive dragging for now