Sprite origin fixes (and more)

[VladiStep]

Room editor changes:

1. Position of rotated/scaled GameObject and SpriteInstance is now properly displayed.
2. Sprite origin fields in the sprite editor now work (with sprite origin wrappers).
3. Added descriptions to some properties (and modified existing).
4. Removed redundant converters: ObjCenterXConverter, ObjCenterYConverter and ObjXOffsetConverter.
5. Added NegateNumberConverter.
6. Suppressed Bitmap-related warnings in "UndertaleCachedImageLoader.cs" and replaced with TODO.
7. "Backgrounds & tile sets" item title changes to "Tile sets" on GMS 2 games.
8. Fixed bug that prevented adding sprite to a new sprite instance.
9. New sprite instances have unique names ("graphic_XXXXXXXX", as in GameMaker Studio: 2).
10. Tiles are now properly updating on their sprite change.
11. Removed GetBGColorLayer() and integrated its contents into BGColorLayer.

Closes #623

[Miepee]

Didnt have time to review everything right now, will review more later

[Miepee]

Second round of reviews