Added 15 more levels - did some UI things

Been meaning to actually add some more levels into the game for a while, been distracted with the level editor and refactoring a bunch of old code. Finally got some time to use the level editor I made so I could make... levels, and added some in.

I also made it so that the default levels aren't shown in the list of custom levels in the editor - they'd seriously clog it up otherwise and you'd have to scrolls through a lot eventually.

Added a button to return to the main game after testing levels too. Quite handy.