Move Look and Feel KCM to top level

Summary:
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.

Reviewers: Plasma, VDG, rooty, ngraham, davidedmundson

Reviewed By: Plasma, VDG, rooty, ngraham, davidedmundson

Subscribers: ngraham, davidedmundson, abetts, rooty, VDG, plasma-devel, Plasma

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D18892