Add ModelLibrary::CreateEmpty().

This is useful for creating procedurally generated models.
Add the beginning of the direction arrow model to EditorMetadataComponent. The default renderer does not yet honour this, but will with the next few commits.
Replace an assert with a proper conditional since that particular case is now permitted.
Add a setting to the editor metadata panel in the scene editor to allow an object to show a direction arrow.

The direction arrow is still not drawn by the default renderer in this commit. Coming in the next commits.
Add support for drawing the direction arrow to the default renderer.
Show the direction arrow on spot and directional lights in the scene editor by default.
Fix a bug where the vertex arrow of the direction arrow in EditorMetadataComponent was not being garbage collected. Instead, it was being deleted directly which means there's a possibility that the rendering thread might try to use it after it's been deleted.
Fix a bug in the scene editor where the editor component settings panel was not updating the "Show Direction Arrow" check box when a scene node was selected.
Add some foundations to the scene editor gizmo for rotation support. Nothing functional yet.
Rename PositionGizmo to TransformGizmo.
Silence VC++ 4355 warning in GameScript.
Add the ability to rotate a SceneObject around the world axis using a pivot point.
Add rotation support to the the transform gizmo.
Improve the picking shapes for the transform gizmo circles.
Improve the rotation circles of the transform gizmo.
Another small improvement to the transform gizmo.
Reduced the resolution of the transform gizmo circles.

This makes the camera movement a little bit more efficient.
An experimental change to the dragging of the circles in the transform gizmo.
Add a separator into the right-click context menu of the scene editor.