This was only done in the new gtk3 backend, all other backends seem to
ignore the GDK_KEY_RELEASE event (especially the gtk2 one). So make the
gtk3 backend code consistent with the other backends.

Change-Id: I3bdecb7ce05190ee2496bc552ca79375fb6fd713
Reviewed-on: https://gerrit.libreoffice.org/17431Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>
(cherry picked from commit a2eaa1bb)
Reviewed-on: https://gerrit.libreoffice.org/17529