Move non-Compositor functions out of composite.cpp source file

Summary:
Compositing today is ubiquitous. There is no reason to keep compositing
specific functions of Toplevel, Client and Workspace classes in the
composite.cpp source file. Instead let these definitions be separated.

Test Plan: Compiles

Reviewers: KWin, zzag

Reviewed By: KWin, zzag

Subscribers: zzag, kwin

Tags: KWin

Maniphest Tasks: T11071

Differential Revision: https://phabricator.kde.org/D21654