MenuStrip UI tweaks

Improved visibility of MenuStrip arrows; more polishing for Tech Demo
Add debug convenience constants

These constants can be used to disable certain features of the game to
speed up the debugging-testing cycle.
Update Fader to use seconds instead of frames

As a side effect, this also fixed the BGM fade in the opening cutscene!
:-)  Note that the Fader continues to work correctly even if the
framerate is changed in the middle of a fade.