Implement basic keyboard binding in RGUI.

I've implemented a basic keyboard binding scheme in RGUI. Still kinda hacky though.

Press RIGHT on binds instead of OK/accept button to bind keyboard. It has a timeout, and if hit, it'll just clear out the bind (can't use a key to cancel because that'd just be treated as a bind :P).

Dunno how to make it clearer, I don't want to add doubled up sets of binds in the menu just to differentiate between things. Maybe a bind mode (keyboard / joypad) is doable, but we'll see.

Binding keyboard stuff from RGUI is obviously not as "safe" as joypad binds. If you end up breaking binds, you might be locked out of RGUI until you cleanup the config manually.

Another caveat, binding the Quit RetroArch button will immediately exit retroarch for obvious reasons. At least the bind is saved properly ...