Rebind function keys

[Former user]

I'd like to be able to rebind the menu and function keys, like is possible on ZDoom. For example I find F8 for quicksaves to be inconvenient, first because I'm used to it being on F6 and secondly because it makes it possible to fat-finger it when wanting to do a quickload (F9).

[Braden Obrzut]

Duplicate of #198.