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.

tests/setup-origin.c

@@ -11,7 +11,7 @@ void
 test_dummy_origin_set_up (OriginFixture *fixture,
                           gconstpointer user_data)
 {
-  GList *origins;
+  GListModel *origins;
 
   test_dummy_provider_set_up (&fixture->parent, user_data);
 
@@ -20,8 +20,7 @@ test_dummy_origin_set_up (OriginFixture *fixture,
 
   origins = calls_provider_get_origins
     (CALLS_PROVIDER (fixture->parent.dummy_provider));
-  fixture->dummy_origin = CALLS_DUMMY_ORIGIN (origins->data);
-  g_list_free (origins);
+  fixture->dummy_origin = g_list_model_get_item (origins, 0);
 }