gtk3: implement Gtk::HeaderBar

GtkHeaderBar is provided in gtk+3 3.10 or later. see: https://developer.gnome.org/gtk3/3.10/GtkHeaderBar.html

Reported by dark-yux in issue #206.
Thanks!
gtk3 test: add a missing assertion
gtk3 test: suppress a warning

    Gtk-WARNING **:Can't set a parent on a toplevel widget