GListStore requires that item-type be derived from GObject, so specify
that the type of the item parameters is GObject so the functions can be
used via gobject-introspection.
Add a scope parameter for the callback used during insert_sorted.
Add g_list_store_insert_sorted() which takes a GCompareDataFunc to
decide where to insert. This ends up being a very trivial function,
thanks to GSequence.
https://bugzilla.gnome.org/show_bug.cgi?id=743927
GListModel is an interface that represents a dynamic list of GObjects.
Also add GListStore, a simple implementation of GListModel that stores
all objects in memory, using a GSequence.
https://bugzilla.gnome.org/show_bug.cgi?id=729351