Use new slot syntax in Compositor class
Summary:
As a preparation for further changes clean up the Compositor
class. First step is to use the new slot syntax and rename
some of the slots.
Includes some other minor code style improvements to the class
as well.
Test Plan: Manually in X and Wayland nested session.
Reviewers: KWin, zzag
Reviewed By: KWin, zzag
Subscribers: davidedmundson, zzag, kwin
Tags: KWin
Maniphest Tasks: T11071
Differential Revision: https://phabricator.kde.org/D22218