imquartz: Fallback to slave IM context if no NSEvent exists.

imquartz: Fallback to slave IM context if no NSEvent exists.

When an application translates a key event and drop its native event
before passing to imquartz, it can't recognize the NSEvent. On this
case imquartz doesn't emit any signals such as "commit" signal so
that the application doesn't insert any text. To avoid no response,
at least imquartz should fallback to slave GtkIMContextSimple.

https://bugzilla.gnome.org/show_bug.cgi?id=694273#c27
(cherry picked from commit c064e188943ad6bd79d8476a0f53e27bfddb4a1f)