Split up fullscreen able check into AbstractClient subclasses

Summary:
Most parts of this function are only relevant for X clients, in particular
the "fullscreen hack". Therefore split up the function into the AbstractClient
subclasses.

Test Plan: Manually and autotests still pass.

Reviewers: KWin, zzag

Reviewed By: KWin, zzag

Subscribers: graesslin, zzag, kwin

Tags: KWin

Maniphest Tasks: T11098

Differential Revision: https://phabricator.kde.org/D18128