v0.2.0.0

Added undoing! It's a pretty important feature, so I'm jumping from 0.1.0.0 straight to 0.2.0.0. You can't stop me.
Undoing can be very bug-prone, so let me know if you find any weird behaviour with it. 

Here's the relatively small changelog:
- Added undoing in the level by pressing Z. You can also manually record an undoable savestate with U (which can be helpful in platformy levels)
- Added a small visual separator between locks in the door editor.
- Fix being able to pull up the keypad from the editor.
- Fixed a bug where keys were being collected one frame too late (nobody would've noticed this one, but it ruined the undo system)