[effects/startupfeedback] Port to KConfigWatcher

Summary:
startupfeedback is a special case, rather than loading our own config we
load settings from klaunchrc.

In order to reload this means leaking kwin specific calls into the
launch feedback KCM. Using the new KConfigWatcher syntax, we can use a
generic solution for all this code.

Test Plan:
Changed the value in KCM
Opened dolphin
cursor did the right thing

Reviewers: zzag

Reviewed By: zzag

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D25834