region: Show the more button when there are extra regions

The more button is not visible, even when the list contains extra items.
This happens because the button is added before the regions are added.
When region_visible is called the list has no other elements so the
button will not be visible. Add the button after all regions have been
added.

This is however not enough. The user might have moved back and selected
a different language and while this will update the list the button will
not be visible. Invalidate the list when the page is mapped.

https://bugzilla.gnome.org/show_bug.cgi?id=729208