D17890: Move groupTransient and group from Client to AbstractClient as virtual

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REVISION 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

REPOSITORY
  R108 KWin

BRANCH
  group-transient-abstract-client

REVISION DETAIL
  https://phabricator.kde.org/D17890

AFFECTED FILES
  abstract_client.cpp
  abstract_client.h
  client.h
  deleted.cpp
  group.cpp
  layers.cpp
  workspace.cpp

To: graesslin, #kwin
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