Made ListSelect value default to 0, not null

Also added ListSelect.getValue(), to get the actual value instead of the
selection's index in the ListSelect's list.
Made radio group default to 0 instead of "none"
Moved level editing features into the canvas

Still rough around the edges but mostly functional. All that's left is
adding level loading and saving, which should be fairly
straight-forward.

A side-effect of this is that the level selector at the bottom of the
screen doesn't work; the start menu will need to be worked on a bit more
to get the game back to a truly working state (also should be fairly
straight-forward)