Move the text for buttons from the Help screen into cnst.py
Modify buttons shown in the Help screen for the GCW Zero
GCW Zero: Add a packaging script and desktop metadata file
GCW Zero: New button map
Fix invisible graphics in the boss level

Tile 0x01, the yellow smiling square, can be created from ending the
first phase of the boss level and it is thus mutable.

This slows down certain levels that use tile 0x01.