Rework ShellClient fullscreen control

Summary:
The ShellClient::setFullScreen method was a direct copy of the Client
version.

Straighten out the function logic now and align with modern coding style.

In short we check:

1. what the manual override window rule wants,
2. if there is a change at all with this,
3. if such a change is possible.

And do:

4. (un-)set the fullscreen,
5. emit the changed signal.

Test Plan: Manually, autotests pass.

Reviewers: KWin

Subscribers: zzag, kwin

Tags: KWin

Maniphest Tasks: T11098

Differential Revision: https://phabricator.kde.org/D18132