RetroArch support

[fenhl]

While supporting all the platforms RetroArch runs on is unrealistic, Windows, macOS, and Linux should be doable. When I last tested it, RetroArch's plugin interface caused extreme ingame lag when used, so selecting RetroArch in the installer should display a warning about that.

[fenhl]

This is now in progress on the retroarch branch.

[fenhl]

If libretro/mupen64plus-libretro-nx#545 gets merged, it would be worth investigating whether it fixes the performance issues.

That PR also includes a link to the docs for the plugin interface which I was previously unaware of.