Update overlay.xul

Per https://developer.mozilla.org/en-US/docs/Extensions_support_in_SeaMonkey_2 , SeaMonkey uses taskPopup instead of menu_ToolsPopup , so add a stanza with the former (I don't know if you can mention two ids for one menuitem). This makes Tools menu show a red[1] TiddlyFox!

[1] To quote skin/overlay.css, "This is just an example. You shouldn't do this." :-)