Move applet packages' /code subdirs below /ui subdirs

Summary:
As there are quite a bit QML files in the respective /ui subdir, having the
JS file in another separate subdir named /code as befire provides
good overview

Test Plan: Applets UI & Config still work as before

Reviewers: #plasma, mart, broulik, hein

Reviewed By: #plasma, mart

Subscribers: apol, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D11148