region: Reuse the input chooser instance

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.