Setting a root tile: will fill in the extension tiles
Deleting a root tile: will delete all extension tiles (it assumes extension tiles are valid)
Add checkbox to prevent overwriting tiles that aren't none (eg if you set a 4x4 and it'd overlap with a 2x2, the program would play a sound indicating that the requested operation was disallowed).
Closes#136 -- multi-select with the current drawing setup isn't really feasible. Bulk set/delete addresses the underlying request.
