Support keyboard hotkeys that GTK+ considers invalid

Support keyboard hotkeys that GTK+ considers invalid

We re-implement the usual key handling by overriding the key press
handler, emulating the according steps but leaving out the
validation that normally rejects certain combinations.

Single key hotkeys work unless an entry is focussed, in which case
the entry receives the input, and Control+Tab works as well.

The shortcuts extension is adjusted to accept any hotkeys.