[effects] Move the Wobby Windows effect to the Candy category

Summary:
The Wobby Windows effect is more a "candy" rather than a standard effect,
so it makes more sense to move it to the Candy category.

Test Plan:
Opened the Desktop Effects KCM, the Wobby Windows effect is in the Candy
category:

Reviewers: KWin, Plasma, VDG, ngraham, broulik

Reviewed By: Plasma, VDG, ngraham, broulik

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D14455