Adjust and remove some top-level categories to implement new Appearance section layout

Summary:
This is patch two out of three in support of the new Appearance section layout in Plasma 5.16 decided upon on in T8871: Systematic KCM reorganisation.

In this patch, the ordering of some categories is changed, and the top-level Workspace Theme category is removed.

Test Plan:

Reviewers: VDG, Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: plasma-devel

Tags: Plasma

Maniphest Tasks: T8871

Differential Revision: https://phabricator.kde.org/D18417