user-configurable hotkey via in-game settings

This makes the existing https://github.com/OpenRA/OpenRA/wiki/Controls user-configurable via in-game settings and adds sell/repair/powerdown hotkey to the RA/D2k mod, too.

![hotkey-config-simple](https://f.cloud.github.com/assets/756669/142118/2ece995e-72c3-11e2-9516-3580957c981c.png)

It is a heavily stripped down version of #2501 which was a clean-up of #2000 initially by @TiriliPiitPiit.