Add in app notifications

This patch adds in-app notifications. The code is derived from
gtk-widget-factory.

https://bugzilla.gnome.org/show_bug.cgi?id=723996
Notifications: Add static notifications

Add support for static notifications. These are pre-defined
notifications that are cached and reused.

https://bugzilla.gnome.org/show_bug.cgi?id=723996