[kcms/launchfeedback] Port to KConfig notifiers
Summary:
This gets rid of all C++ code in this KCM!
It gets rid of a kwin tie in where one shouldn't exist and also exposes
a notification that the taskmanager should be able to hook into -
something currently missing as it loads this setting.
Test Plan: Changed setting, kwin updates automatically
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D25835