app chooser: Avoid duplicates

At the time we populate the model "initially" in constructed(),
it has already been filled and cleared a couple of times (we do
that every time one of the construct properties gets set). So
we can't assume that the model is empty, and have to clear it
first. Otherwise, we add duplicates to the list.

https://bugzilla.gnome.org/show_bug.cgi?id=748080

gtk/gtkappchooserwidget.c

@@ -813,7 +813,7 @@ gtk_app_chooser_widget_initialize_items (GtkAppChooserWidget *self)
                 NULL);
 
   /* populate the widget */
-  gtk_app_chooser_widget_real_add_items (self);
+  gtk_app_chooser_refresh (GTK_APP_CHOOSER (self));
 }
 
 static void