Level editing, done

It’s taken a bit to get back into the post-holiday routine, but I’m getting there. I’ve completed the “edit levels” side of the editor, and now I just have to make it so that you can toggle into play mode.

New additions in this build:
- cmd+r resets the room
- clicking on doors lets you pick the room file that the door should exit to
- You can only place the player and goal tiles once

I spent a lot of time trying to come up with the best UI pattern for door exits, which was frustrating - after giving it a lot of thought it turned out that the best pattern was just to have it be a file select dialog (no extra UI chrome needed). Oh well - I know for next time, at least.