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.
Migration is still TODO
For the compositing KCM, I was going to port it to use the new location and then
hide the slider if we're on a different desktop to Plasma.
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.