When pressing a key which doesn't exist as Qt.Key, we don't get Qt.Key_unknown
like we'd expect, but we get 0x0 instead...
Let's add that as a new "nil" key (to not conflict with None/unknown/zero/...)
and handle it appropriately.
This can be reproduced by doing:
setxkbmap -layout us,gr -option grp:alt_shift_toggle
and pressing Alt-Shift/Shift-Alt.
d1854eddaf