avahi-ui: Remove deprecated usage of gtk_widget_push_composite_child/gtk_widget_pop_composite_child
avahi-ui: Remove deprecated usage of gtk_widget_push_composite_child/gtk_widget_pop_composite_child

The composite API is deprecated in GTK3, it doesn't really do anything other than
hide the container objects from gtk_container_foreach which no one is really doing
with avahi-ui that I can find anyway, so we remove it entirely rather than make it
conditional.

Ideally avahi-ui should be converted to GtkBuilder where there is a new composite API

lathiat/avahi,lathiat/avahi,heftig/avahi,heftig/avahi,heftig/avahi-1,lathiat/avahi,heftig/avahi-1,heftig/avahi-1,heftig/avahi-1,heftig/avahi,heftig/avahi,lathiat/avahi,lathiat/avahi,heftig/avahi,heftig/avahi-1,heftig/avahi-1,heftig/avahi,lathiat/avahi