call-window: allow user to enter dtmf events using his keyboard

Ideally we should stop sending the DTMF event when the key is released but we
can't easily do that in Gtk+; we'll have to write our own widget.
I'm going to give it a shot but this is already a nice improvement so we
shouldn't block on it.

https://bugzilla.gnome.org/show_bug.cgi?id=679396