notifications: Fix order of title/banner for RTL text

Currently, the banner text is always located at the right of the
title - we need to swap those around for RTL locales. Rather than
using the locale directly to figure out the ordering, try to
determine the direction of the title label and let it control the
overall direction of the notification - this way, notifications
generally come out right when mixing scripts.

https://bugzilla.gnome.org/show_bug.cgi?id=646921