gtkcellrendereraccel: Use a GtkInvisible to grab on

gtkcellrendereraccel: Use a GtkInvisible to grab on

Grabbing on a non-toplevel might not do what we want it to do, since it
will go on the focused widget, not the grabbed widget. Since we don't
focus the widget before clicking on it, that means that putting the
focus somewhere else and then clicking on the accelerator editor will
freeze the app. Additionally, since it's a global system grab that can't
be exited except by a key press that we won't ever get, it effectively
locks up your system as well unless you know how to break the grab or
kill the app. Ouch.

Since doing a device grab on a non-toplevel is generally considered a
bad idea, just don't do it. Use a GtkInvisible and take a grab on that
instead.