The language input chooser is being created and destroyed every time
it is opened and closed. This is noticeably slow and is hardly
necessary as the values it contains should not change.
To fix this, these changes reuse the same instance of the input chooser
(by hiding and showing it) instead of renewing it.
The input chooser is also now shown using gtk_dialog_run to avoid having
it destroyed from the default delete event.
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
