Generalize constrainedStackingOrder for group transients and ordinary transients

Summary:
We don't really have to have two different code paths for group
transients and ordinary transients. For now, AbstractClient::hasTransient
is good enough to check the relationship between potential parent and
the transient.

In long term, we need to "invert" the relationship, instead of checking
whether given parent window has a transient, we should check whether
given transient is a transient for a given window so we can keep Deleted
transients above their old parents.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D15893