Merge applet packages' /code subdirs into /ui subdirs

Summary:
As there are only a few QML files in the respective /ui subdir, having the
JS file in the same dir still provides good overview

Test Plan: Applets UI & Config still work as before

Reviewers: #plasma, mart

Reviewed By: #plasma, mart

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D11144