[kcmkwin/desktop] Add animation option back

Summary:
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.

Test Plan: {F6503565}

Reviewers: #kwin, #vdg, ngraham, davidedmundson

Reviewed By: #kwin, #vdg, ngraham, davidedmundson

Subscribers: davidedmundson, hein, ngraham, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D17766