Use virtual modifier <Super> for the Windows key

By making sure that we convert virtual modifiers to real ones when
capturing keys with XGrabKey, and vice-versa when checking for
XEvents.

This is based heavily on a patch from

Bastien Nocera <hadess@hadess.net>

with just minor fixes.

https://bugzilla.gnome.org/show_bug.cgi?id=623223