Gtk: Scrub resource:// prefix for template path

Gtk.Widget.bind_template_child_full expects an absolute path to the
resource. We need to scrub the resource:// part of the template string.

https://bugzilla.gnome.org/show_bug.cgi?id=739739
Gtk: Add test case for template from resource

https://bugzilla.gnome.org/show_bug.cgi?id=739739