Flexible composite swap and timer events

Summary:
The GLX backend might need a combination of swap and composite timer events for
continous painting.

The reason for that is that if the buffer age extension is not available we
fall back to copies in case not the whole screen is repainted.

The timer logic is adapted to make this possible in a lean way what cleans up
the Compositor class in several ways.

Test Plan: Tested on X11 (with/without swap events, buffer age enabled)  and Wayland.

Reviewers: #kwin

Subscribers: hurikhan77, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D26216