Load animation speed from new global animation speed controller

Summary:
It doesn't belong with advanced compositing settings as it's quite user
friendly, and we also want to adjust other animation speeds. May as well
do it together.

In the current form all compositing is still completely reinitiliased
like with the previous slider. Change notifications come in the form of
KConfigWatcher rather than our own bespoke update interface.

Test Plan:
Moved new slider, minimised a window.
It still behaved as expected.

Reviewers: #kwin, zzag

Reviewed By: #kwin, zzag

Subscribers: zzag, broulik, anthonyfieroni, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D22887