scripting: Introduce ClientModelByScreenAndActivity

Summary: Provide for KWin scripts the possibility to work with a activities aware ClientModel. In the same way that the ClientModelByScreenAndDesktop works with virtual desktops.

Reviewers: #kwin, mart, zzag

Reviewed By: #kwin, zzag

Subscribers: zzag, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D29284