Bump Gtk+ required version to 3.13.4.

https://bugzilla.gnome.org/show_bug.cgi?id=735806
Use GResource for loading appmenu.

https://bugzilla.gnome.org/show_bug.cgi?id=735806
Use set_accels_for_action().

https://bugzilla.gnome.org/show_bug.cgi?id=735806
Reorder calls to ensure window uniqueness.

https://bugzilla.gnome.org/show_bug.cgi?id=735806
Remove unused namespaces.

https://bugzilla.gnome.org/show_bug.cgi?id=735806
Clean the grid saving when quitting application.

https://bugzilla.gnome.org/show_bug.cgi?id=735806