Add GListModel

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

gio/giotypes.h

@@ -61,6 +61,8 @@ typedef struct _GPermission                   GPermission;
 
 typedef struct _GMenuModel                    GMenuModel;
 typedef struct _GNotification                 GNotification;
+typedef struct _GListModel                    GListModel;
+typedef struct _GListStore                    GListStore;
 
 /**
  * GDrive: