Fixed "Bug 21831 - Control-shift-z no longer works".
I needed to allow alternate key bindings for commans to fix that
issue. Command:KeyBinding is a 1:n relation and no 1:1. I didn't break
the API with that change but it would be nicer if the command would
only have a KeyBinding[] property instead of the splitted key
bindings/alt bindings. To design is how (or should?) we allow users to
set alternate key bindings as well.