Move core shade code to AbstractClient

Summary:
In order to allow shading wayland clients, this change moves core shade
code from X11Client to AbstractClient.

Test Plan: Shading still works on X11.

Reviewers: #kwin, cblack

Reviewed By: cblack

Subscribers: cblack, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D29512