[frontend] kill hardcoded hotkeys

[Jaklyy]

kill them all (savestate save/load/undo and open rom)
also add hotkeys for every single thing you can think of while you're at it

[TheRealWario]

As someone who fucking hated rpcs3’s old built-in and unchangable hotkeys, I fully support this.

[AlmightyFrog]

A little beyond hotkeys, but e.g. a hotkey which uses always the next slot in comparison to previous save would be cool.

I liked the retro arch way of savestates where you can endless just always save in new file and then skip some back in case you need it.
Having 10 savestates history is better than one only.

Edit: see also #2194 for even better having save hotkey also on gamepad.