region: Don't show IBus sources in the system-wide input chooser

Since we can't export IBus sources through the localed API we
shouldn't even allow users to choose them.

This allows us to stop having to show an apologising dialog which
makes us look a bit bad and was using deprecated gtk+ API that we're
trying to get rid of. Instead, we show the same information up front,
in the input chooser.

https://bugzilla.gnome.org/show_bug.cgi?id=740986