app: let the tag popup own its tag objects

This should fix crashes in which tag objects got accessed after they
were disposed, because they still were stored with the popup object.
(cherry picked from commit b109e058)

app/widgets/gimptagpopup.c

@@ -257,6 +257,8 @@ gimp_tag_popup_constructed (GObject *object)
       tag_data->tag   = tag_iterator->data;
       tag_data->state = GTK_STATE_NORMAL;
 
+      g_object_ref (tag_data->tag);
+
       for (j = 0; j < current_count; j++)
         {
           if (! gimp_tag_compare_with_string (tag_data->tag, current_tags[j]))
@@ -386,6 +388,13 @@ gimp_tag_popup_dispose (GObject *object)
 
   if (popup->tag_data)
     {
+      gint i;
+
+      for (i = 0; i < popup->tag_count; i++)
+        {
+          g_object_unref (popup->tag_data[i].tag);
+        }
+
       g_free (popup->tag_data);
       popup->tag_data = NULL;
     }