Creation tweaks (to Brian's branch)

This is for Brian's creation mode.

In particular:
* The custom level appears in the level select screen on the last page, and it is always unlocked.
* The floating "Collision!" text is gone, and it is replaced with something a bit less debug-y in the lower left corner.
* The lower left corner's text is backed by a partially transparent rectangle.
* Fix a bug in LevelGroup.
* Some minor changes to certain strings.

@bmarroquin Can you take a look at this real quick?