front: Enhanced the multi-player mapping UI.

Player mappings persist. UI is in the main menu under Play since it
will need to be accessed often, right before launching the emulator.
front: Code cleanup, variable renaming.
front: More informative player mapping screen.
bugfix: Multi-player works even if Player 1's controller disabled.

Bug first reported at
http://www.paulscode.com/forum/index.php?topic=723.msg7581#msg7581