gtk+-3.0: fix flowbox selection foreach API

The callback doesn't have a destroy notify, so don't mark it 'owned'.
Additionally, it's not OK to pass a NULL callback to a foreach function.

https://bugzilla.gnome.org/show_bug.cgi?id=728474
gtk+-3.0: add new GtkListBox selection API

metadata copied from the similar api on GtkFlowBox (which is where the
GtkListBox API itself was modelled from).

https://bugzilla.gnome.org/show_bug.cgi?id=728474