region: Don't ever show all input sources in the chooser

While filtering we're adding all the input source rows to the
GtkListBox. This is a performance issue since GtkListBox doesn't
really cope with a big number of rows.

Instead, we'll filter the locale rows in a way such that we'll show a
locale row for which any input source name matches. From this filtered
locales view, users will still need to activate a locale row to get to
the input source rows view.

https://bugzilla.gnome.org/show_bug.cgi?id=752383