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