GListModel support

GtkFilterListModel — a GListModel that filters its items
GtkFlattenListModel — a GListModel that flattens a given listmodel
GtkMapListModel — a GListModel that maps items from a child list model to a different item
GtkSliceListModel — a GListModel that presents a slice out of a larger list
GtkSortListModel — a GListModel that sorts a listmodel
GtkTreeListModel — a List model that can create child models on demand
GtkSelectionModel — An extension of the list model interface that handles selections
GtkSingleSelection — A selection model that allows selecting a single item