[kcmkwin/desktop] Add animation option back
The "new" animation option no longer uses hard coded effects, which
means one could install a third party virtual desktop switching animation,
for example from store.kde.org, and it will be displayed in the KCM.
Subscribers: davidedmundson, hein, ngraham, kwin
Differential Revision: https://phabricator.kde.org/D17766