It turns out that simply using XIGrabKeycode instead of XGrabKey
makes the DND keyboard support mostly work (there seem to be some
minor issues with modifiers). This means we no longer grab the
keyboard actively during DND, which in turn makes Alt-Tab and
other window manager shortcuts work again during DND.

At the same time, bring the DND key handling code into the
multi-backend work, by checking for X11 and XI2 at runtime,
in addition to compile time.