Move groupTransient and group from Client to AbstractClient as virtual

Summary:
The default implementations just return false/nullptr. The advantage of
having this in AbstractClient is that we can reduce the needed casts
from AbstractClient to Client in core as can be seen in this change.

There are more cases which can be improved thanks to this refactoring
which will follow in dedicated commits.

Test Plan: ctest passes

Reviewers: KWin

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D17890