Work around access to global resources without calling gtk_init()

Work around access to global resources without calling gtk_init()

Some utilities such as GIR and gtk-doc, initialize class vtables without
initializing GTK+, with composite templates accessing resources this
causes a hand full of unneeded warnings.

The workaround for now is the use a private function _gtk_ensure_resources()
which is both called while initializing GTK+, and at the beginning of
gtk_widget_class_set_template_from_resource() (the private function
ensures that the resource will only ever be registered GOnce).