provider: Use GListModel to track origins

And adapt to changes

It's easier to track changes if we are using GListModel than
a GList.  Also, eventually we shall have multiple providers,
and we can use GtkFlattenListModel to merge multiple origins
without manually managing pointers ourself.

Since several widgets has APIs to handle GListModels, it will
also help us create widgets from GListModels easily.

src/calls-provider.c

@@ -58,13 +58,6 @@ enum {
 static GParamSpec *props[PROP_LAST_PROP];
 
 
-enum {
-  SIGNAL_ORIGIN_ADDED,
-  SIGNAL_ORIGIN_REMOVED,
-  SIGNAL_LAST_SIGNAL,
-};
-static guint signals [SIGNAL_LAST_SIGNAL];
-
 static void
 calls_provider_get_property (GObject    *object,
                              guint       prop_id,
@@ -99,20 +92,6 @@ calls_provider_class_init (CallsProviderClass *klass)
                          G_PARAM_READABLE | G_PARAM_STATIC_STRINGS);
 
   g_object_class_install_properties (object_class, PROP_LAST_PROP, props);
-
-  signals[SIGNAL_ORIGIN_ADDED] =
-    g_signal_new ("origin-added",
-                  G_TYPE_FROM_CLASS (klass),
-                  G_SIGNAL_RUN_LAST,
-                  0, NULL, NULL, NULL,
-                  G_TYPE_NONE, 1, CALLS_TYPE_ORIGIN);
-
-  signals[SIGNAL_ORIGIN_REMOVED] =
-    g_signal_new ("origin-removed",
-                  G_TYPE_FROM_CLASS (klass),
-                  G_SIGNAL_RUN_LAST,
-                  0, NULL, NULL, NULL,
-                  G_TYPE_NONE, 1, CALLS_TYPE_ORIGIN);
 }
 
 static void
@@ -151,10 +130,9 @@ calls_provider_get_status (CallsProvider *self)
  *
  * Get the list of #CallsOrigin interfaces offered by this provider.
  *
- * Returns: A newly-allocated GList of objects implementing
- * #CallsOrigin or NULL if there was an error.
+ * Returns: (transfer none): A #GListModel of origins
  */
-GList *
+GListModel *
 calls_provider_get_origins (CallsProvider *self)
 {
   g_return_val_if_fail (CALLS_IS_PROVIDER (self), NULL);