Remove the "Candy" category and fold its contents into "Appearance"

Summary:
The "Candy" category suffers from some problems:
- It does not need to exist; everything in it is purely appearance-related and could live in the existing "Appearance" category
- The name is inappropriate; "Eye Candy" would be better, but changing it to this has proven controversial in the past

Let's just remove the category and move everything in it into the "Appearance" category.

Test Plan:
Apply and compile
Open Desktop Effects KCM
See that the "Candy" category is no more and the "Appearance" category has more stuff in it

Reviewers: #kwin, #vdg, zzag, abetts

Reviewed By: #kwin, #vdg, zzag, abetts

Subscribers: abetts, zzag, kwin, kde-doc-english, GB_2

Tags: #kwin, #documentation

Differential Revision: https://phabricator.kde.org/D27658