actionmuxer: avoid duplicate and recursive work
If we've already done the tracking into the parent muxer, there is no need to do it again. This can save a great deal of recursive work when adding items to the muxer.
This makes showing the context menu in gnome-text-editor repeatedly fast even as spelling corrections are changed.
It is likely that this could fix #4422 (closed) as well.
Fixes #4519 (closed) Fixes gnome-text-editor#220 (closed)