widget: Use a real offset in gtk_widget_class_automate_child

widget: Use a real offset in gtk_widget_class_automate_child

Using an offset from the struct means you can have children in
both the public and private (via G_PRIVATE_OFFSET) parts of the
instance. It also matches the new private macros nicer.

https://bugzilla.gnome.org/show_bug.cgi?id=702563

Signed-off-by: Emmanuele Bassi <ebassi@gnome.org>