Implement Apply desktop rule for ShellClient

Summary:
This change sets up ShellClient for supporting window rules by reading
in the rules once it gets created. As a first rule the Apply initially
rule for desktop is implemented.

Currently it is not yet possible to set window rules through the
configuration menu. So far only injecting rules through the test
framework (temporary rules) is implemented. The idea is to first
implement all rules then to expose them to the UI.

Test Plan: New test case

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D8177