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