Move Look and Feel KCM to top level
Moves the LnF KCM from the Workspace Theme category to the top level (to the Appearance section), because:
- the KCM affects everything, not just workspace themes
- then it's faster to discover the KCM and go to it
It won't affect any other KCMs, so this change should be fine.
Also see T8871.
Test Plan: Open the system settings.
Differential Revision: https://phabricator.kde.org/D18892