glade-gtk-searchbar.c: Avoid critical warnings when loading searchbars

Changed glade_gtk_search_bar_add_child() to not make the assumption
that a current "child" widget exists, which it doesnt at project load
time (until one is actually loaded).
glade-gtk-widget.c: Minor fix for 'add_parent' action

Previous implementation assumes that the parent widget can
be retrieved with gtk_widget_get_parent(), use glade_widget_get_parent()
instead and trust the project datamodel instead, as the logical
widget hierarchy may not be exactly the truth, as is the case for
a searchbar child.