Stacking Order becomes a list of Toplevel Windows

The common usage of stacking order is to loop through
the list and find a specific Client. All these usages
still need to find a Client. For that the loops are
adjusted to first cast the Toplevel into a Client and
continue if the current item is no Client.

At the moment all entries in the stacking order should
still be Clients as the Deleted are not yet added.