[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:

{F6160383, layout=center, size=full}

Reviewers: #kwin, #plasma, #vdg, ngraham, broulik

Reviewed By: #plasma, #vdg, ngraham, broulik

Subscribers: kwin

Tags: #kwin

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