Support GtkSearchBar

This widget has been around since 3.10, high time that glade
starts supporting it.

https://bugzilla.gnome.org/show_bug.cgi?id=738493
Searchbar: Ignore some properties

Don't apply the search-mode and show-close-button properties
in the embedded view, since it would be confusing to have the
search bar just 'disappear'. We do save these properties, so
it is possible to set them up for the actual application.
Add a get_children function for GtkSearchBar

This is needed to make saving children of GtkSearchBar work.
gtk_container_forall is sdaly broken for many containers.
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.