MainWindow: remove unnecessary GtkGrid

Make the window-content object be a GtkOverlay instead of a GtkGrid and
hence push the creation of the overlay to the UI-file.

https://bugzilla.gnome.org/show_bug.cgi?id=723996
MainWindow: move the zoomcontrol to here

Initialize the zoom control outside of the mapView, just like the
searchpopup.

As a side effect this removes the circular dependency
between the map overlay and the map view.

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

Two stubs for example static (reusable) notifications. Not meant to be
pushed as is, but as a demonstration on how to work with this.

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

With this applied, try pushing the location button or changing base
layer.

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