Rewrite the input handling

[olofson]

As of 0.7.x, there are basically two separate input handling subsystems; one for the UI and one for the actual game. Both of these have various issues;

• Limited configurability. (Basic key binding config added recently, but that's all there is.)
• Two different subsystems!?
• The game input handler does parts of the decoding/combining on the wrong level, so for example, we can't temporarily disable mouse control without resetting and losing "stick" direction, regardless of actual source.
• UI input lacks key repeat emulation. (This was once handled by SDL 1.2, but SDL2 doesn't have that feature.)

Maybe simply grab the input code (with config GUI) from Eelium/Kobo II, after incorporating EEL? Bonus: If people want really weird input configs, they can hack the scripts.