workspace: Use bounding-box property from layout manager

workspace: Use bounding-box property from layout manager

Make use of the new bounding-box property we introduced for the
WindowClones layout manager in the last commit.

With this we can remove all the bounding box calculation code from the
WindowClone class and simply use the "notify::bounding-box" signal to
notify changes to our size. To make sure users of the WindowClone don't
break, we now have to convert the layout managers ClutterActorBox in our
getter function to a JS object.

Since we now also don't have to connect to the "destroy" signal of the
attached dialogs anymore, we can remove _disconnectSignals() and only
listen to "destroy" of the toplevel window.

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1297