Major Cleanup and Instantiation

MapEditorWindow:
- added functionality to draw/erase, change floor, show prefab
- changed functionality of save/load/save/save as/clear
- added sprites for UI window (eraser, pencil, up, down)

InstanceManager
- added functionality to instantiate things

Map / MapObject / MapChange
- completely removed undo/redo functionality
- completely removed change history

Too many changes honestly. Had to fix a lot of things that were broken.
Loading and Saving works. We can place things on the map.

Bugs:
- lots of problems with assembly reloads disconnecting prefab/instance
managers from game objects.
- some bugs with the editor UI (save/clear/new)

TODO;
- add erase functionality
- fix some of the assembly reload bugs (this won't be an issue when the
editor is done)
- add functionality to render things in the correct order
- add functionality through the instance manager to keep track of
stacking