Gtk: Add overrides for Gtk.Assistant
Gtk: Add overrides for Gtk.Assistant

Overrides for _container_add and child property access, overcoming brain-dead
design of GtkAssistant widget, which is a window descendant, instead of being
generic GtkContainer descendant with direct native child and child properties
access.

zevv/lgi,psychon/lgi,pavouk/lgi