Introduce AbstractClient::destroyClient()

Summary:
The new method provides a generic way for destructing clients. Notice
that we can't just delete clients because we may need to discard
temporary window rules, which is usually done in destroyClient().

Test Plan: Compiles.

Reviewers: KWin, davidedmundson

Reviewed By: KWin, davidedmundson

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D27821