More refactoring on the frackin tile system
Tile painting now correctly goes through the resource manager. No more
pointers to internal model structures littered about.
Undo now works, but after that you can't paint any more :(
Undo now works, kinda
ModifyTilesCommand now stores a list of changes
Removed the add tiles command.
Undo/redo now works, after a painful refactor. Still crashes some though
Preview now clears correctly
Undo/redo now works well for tile painting. Resource addition and
deletion is another matter
Merge branch 'UndoFixes' of https://github.com/Preece/TGL into UndoFixes