[tabbox] Fix casts to Client where it should be AbstractClient

Summary:
A few areas in TabBox perform casts from Toplevel to Client. By casting
to Client instead of AbstractClient the Wayland windows are excluded.

This change addresses the problem by changing the casts where possible.
The only remaining cast to Client is for shading which is not (yet)
supported for Wayland windows anyway.

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D6273