Update: end and fill in \res\sounds

Cut extraneous silence off the end of end.ogg, and faded out volume on
fill.ogg so it doesn't end so abruptly.
Add/Update: Credits sequence implemented.

Moved congrats out of outro and into its own file, congrats.py. The
combination of congrats and credits in one module made for too large and
complex a module for my taste.

Main and ui\__init__ also updated accordingly to run credits.

Game is now essentially back to the functionality of the master branch.