[effects] Move Fall Apart and Sheet to the Candy category

Summary:
The Fall Apart effect and the Sheet effect look like "candies" so
let's move them to corresponding category.

Test Plan: {F6167489, layout=center, size=full}

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

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

Subscribers: ngraham, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D14517