More (shitty) editor functionality

README:
* swap between game & editor with tab
* when entering editor, move up/right first (camera set up funny)
* Q and W alter block type
* P toggles paint mode, move around and paint stuff
* , and . toggle layer

For some reason edits above layer 1 are not shown.

New stuff:

* Block: added moveTo(), setLayer() and setType()
* Camera: added moveTo(), getPosition()
* Cell: getPosition()
* Editor: fuckton of new stuff
* Graphics: added loadIdentity() (not actually used anywhere)
* Vec2i: added operator+
* Vec3ui: added operator+
* World: added getCellAt()