app: fix dragging color boxes into item dockables.

Commit 83b3d9e5 broke dragging color widgets into channels dockables or
other item dockables.
Thanks to Massimo for raising that these lines were needed for this.

app/widgets/gimpcontainertreeview-dnd.c

@@ -540,6 +540,13 @@ gimp_container_tree_view_drag_drop (GtkWidget             *widget,
           gtk_drag_finish (context, success, FALSE, time);
           g_list_free (src_viewables);
         }
+      else
+        {
+          /* Necessary for instance for dragging color components onto
+           * item dialogs.
+           */
+          gtk_drag_get_data (widget, context, target, time);
+        }
 
       return TRUE;
     }