[libkwineffects] Port AnimationEffect to TimeLine

Summary:
TimeLine has nice API for controlling its direction that can be re-used
later by AnimationEffect.

Test Plan: The existing tests for scripting effects still pass.

Reviewers: #kwin, davidedmundson, graesslin

Reviewed By: #kwin, davidedmundson, graesslin

Subscribers: davidedmundson, abetts, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D16448