Add keyboard/rebindOnMappingNotify option

I have to switch to a qwerty layout to play steam games, and every time
I switch back and forth, we lose some random bindings so I also had to
reconfigure Openbox every time, now I don't.
Make RaiseLower work for the dock as well

As a side effect, _NET_RESTACK_REQUEST now also allows specifying the dock as the sibling.