app: in GimpPanedBox, don't accept drag contexts with unsupported targets

... to avoid highlighting the droppable areas during darg.

(cherry picked from commit e38010b2d1)

app/widgets/gimppanedbox.c

@@ -296,6 +296,12 @@ gimp_paned_box_get_handle_drag (GimpPanedBox   *paned_box,
       return FALSE;
     }
 
+  if (gtk_drag_dest_find_target (GTK_WIDGET (paned_box), context, NULL) ==
+      GDK_NONE)
+    {
+      return FALSE;
+    }
+
   gtk_widget_get_allocation (GTK_WIDGET (paned_box), &allocation);
 
   /* See if we're at the edge of the dock If there are no dockables,