Group transients and "ordinary" transients behave differently with respect
to activation:
- If an ordinary transient is activated, its main windows will be raised with it;
- If a group transient is activated, only it will be raised, its window group won't be raised.
That difference exists because transientFor method is meaningless for
group transients, i.e. it returns nullptr.
This change addresses the problem stated above by using an appropriate
method to get a list of main clients.
lowerClient will be addressed in another patch.