Remove some unneeded model files.

This is part of an ongoing effort to reduce the size of the engine
distribution. The idea is to keep the engine distribution as small and
simple as possible and to later have a sort of "asset pack" that people
can download separately if they so desire. I want to push the small size
and simplicity of the engine as a "feature".
Add a Game.CollisionGroups enumerator in preparation for adding support for ray-casting via the scripting environment.
Cleanup.
Add very basic support for ray testing to the scripting environment.
Fix a deserialization bug where the collision filter was not being set.
Attempt a fix for a model importing issue.
Fix a bug where doing Game.Pause/Resume while playing a scene in-editor would not actually pause the scene.
Fix a bug where new scene nodes are not instantiated correctly when created from the scripting environment.
Fix a bug where the mouse is not released when the editor is opened.
Add the ability to recursively retrieve the ID of a descendant scene node via the scripting environment.
Add some slightly better support for random number generation to the scripting environment.
Try to fix a memory leak.
Fix a few crashes with null textures.
Improve the use of Game.Pause and Game.Resume when playing an animation from the editor.
Add support for retrieving the lifetime of emitters of a particle system.
Improve support for ray testing a scene via the scripting environment.
Fix a nasty bug where loading an image in the image editor would cause the image's relative path to be set to it's absolute path.
Fix a few physics bugs.
Fix a bug where material parameters are not correctly getting set.
Update CHANGES.