KCM launchfeedback : port to KConfig XT

Summary: Simple KCM, all settings' states are automatically handled thanks to ManagedConfigModule.

Reviewers: #plasma, ervin, bport, mart, davidedmundson

Subscribers: broulik, ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D25290