RGUI Inputconfig

Here's basic inputconfig for RGUI. This should cover the basic needs for now.

- Can configure all regular RetroPad binds.
- Can configure RGUI Menu Toggle (the most important hotkey).
- Can configure all RetroPad binds in one go (zSNES-like).
- Can default/clear out all binds for a player.
- Joypad index is resolved automatically when you configure anything for a player.

Missing:
- Config for keyboards. This will be quite tricky to implement in a non-ugly way. I've added keyboard redirection to RGUI with libretro keyboard callback, so keyboard centric features can probably work out eventually.
- All other hotkeys than RGUI toggle. Will have to find to a way to expose it without cluttering the UI to hell.