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
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.