Add delay between key presses for some hotkeys

[deltabeard]

• Seek commands should be press and hold to continue seeking. However, a small delay should be added to reduce the delay between when each seek is executed so that the user has enough time to decide when to stop seeking.

• The first delay should be long, and subsequent delays much shorter.

• Other commands should be executed for each key press only. So when R is pressed, the subtitles should only be cycled once and not constantly until the key is released.

This is default mpv behaviour that I would like replicated in mpv-libretro.

[deltabeard]

Made some progress with the last todo item with f50d33a