Use AbstractClient instead of XdgShellClient wherever possible

Summary:
Currently, we have only one shell client type - XdgShellClient. We use
it when we are dealing with Wayland clients. But it isn't really a good
idea because we may need to support shell surfaces other than xdg-shell
ones, for example input panel surfaces.

In order to make kwin more extensible, this change replaces all usages
of the XdgShellClient class with the AbstractClient class.

Test Plan: Existing tests pass.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: davidedmundson, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D27778