[kcmkwin] Don't discard unsaved changes when reloading effects model

Summary:
If an effect is installed or removed, then all not yet committed changes
will be lost. This is undesired behaviour.

Reviewers: KWin, davidedmundson

Reviewed By: KWin, davidedmundson

Subscribers: davidedmundson, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D18705