Bump Gtk+ required version to 3.13.4.

https://bugzilla.gnome.org/show_bug.cgi?id=735053
Use GResource.

https://bugzilla.gnome.org/show_bug.cgi?id=735053
Let startup() do its work instead of activate(), and reorder.

https://bugzilla.gnome.org/show_bug.cgi?id=735053