[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