Make QPA think the tabbox is the active window

Summary:
Without pretendingn to activate the Window, Qt will not send
the right signals for accessibility, thus screen readers not working.

Reviewers: kwin, davidedmundson

Reviewed By: davidedmundson

Subscribers: graesslin, davidedmundson

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D16664