Add default controller hotkey bindings for rewind

[busywait]

RetroArch offers the ability to rewind gameplay in many cores. Rewind is not enabled by default in RetroPie for performance reasons. On my Pi4, many cores are able to use this feature with no apparent issues.

It can be tried easily by enabling Rewind in the RGUI. It can be enabled by default with the (almost obvious) addition of

rewind_enable = "false"

to the relevant retroarch.cfg file for the cores you want to be able to rewind.

This will enable rewind by pressing and holding "r" on the keyboard.

It would be helpful if the joypad button or axis binding for my controller was included in the joypad autoconfiguration file that RetroPie creates. This could be done without rewind being enabled, and would just make it easier to use when it is enabled.

I had to learn a little about the input configuration file syntax to get this right:

For example, I've added these lines for my "SNES-style" D-pad

input_rewind_axis = "+1"

And for my wired Xbox controller

input_rewind_btn = "h0down"

Slightly related, fast forward bindings were a little different again because there are hold vs toggle variants. I bound D-pad up arrow - hold down to fast forward.
For example, I've added these lines for my "SNES-style" D-pad

input_hold_fast_forward_axis = "-1"/

And for my wired Xbox controller

input_hold_fast_forward_btn = "h0up"