Add the possibility to change key mapping

[tkr-sh]

I really like this project, and I think that it would be cool to be able to change the key mapping. For people that don't use QWERTY keyboards, it can be frustrating to play with the controls. So I made it possible to change them.

I tried to keep the UI basic and in coherence with the application.

It's also possible to remap multiple bass/chords to one key. So that you can play multiple sounds by just pressing one key. I don't know if it's a good idea tho.

Hoping that the changes are coherent.