Move WindowRules from Client to AbstractClient

Summary:
So far window rule specific functionality was only implemented in Client
and in ShellClient there was only a dummy implementation. This change
moves the client_rules member variable from Client to AbstractClient.
Areas which would not compile anymore are adjusted.

This is a first step to get window rule support for Wayland windows.

Reviewers: KWin, Plasma

Subscribers: plasma-devel, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D8090