Fix a regression in GtkAssistant child property handling

Fix a regression in GtkAssistant child property handling

GtkAssistant is bending the rules about child properties
of non-direct children, and the recent fix to accomodate
GtkInfoBar changes broke things. The effect was that child
properties of assistant pages in ui files were just not
applied, so all pages ended up without titles and with
the normal page type, leading to broken assistants all
over the place.

https://bugzilla.gnome.org/show_bug.cgi?id=706756