Do axis keymap as well (should replace right stick bind)

Only tested with onscreen controls and DirectInput currently.  Made sure keyboard was still usable even with controller mapped to buttons.

It took me a sad amount of time to realize I needed 5f382c5ab04fbcfbbae57f5f41eb271e0f5ffb1b.

Anyway, it should work well, and also allows people to reverse the analog direction.  There's still todos:
 * accelerometer should probably be an axis?
 * XInput may not do the mapping right and still has the right stick bind stuff.
 * PSP right stick has no UI for mapping, but the code supports it.

Fixes #2503.

-[Unknown]