Document and complete the key remapping codes

[micheldebree]

Is your feature request related to a problem? Please describe.
I cannot remap a function to / and * on the numpad in user.ini

Describe the solution you'd like
I want to be able to assign functions to any key I like, and use numpad keys as separate keys.
I want a list of the keycodes I can use for that.

Describe alternatives you've considered

Additional context

[micheldebree]

I suggest we use https://wiki.libsdl.org/SDL_Keycode and use the same names, remove the SDLK_ bit and make it lower case, for example:

@slash maps to SDLK_SLASH
@kp_multiply maps to SDLK_KP_MULTIPLY

This saves us some thinking, and we can refer to the SDL_Keycode documentation instead of documenting our own codes.
Saves typing aswell if we take the SDL keycodes and do some clever copying en replacing.