[kcmkwin/kwindecoration] Set current decoration index on start

Summary:
Fixes the preselected decoration style on module load.

Also:
* Prevent the module state to be set to modified on resize.
* Fix QML errors in logs.

Reviewers: davidedmundson

Reviewed By: davidedmundson

Subscribers: davidedmundson, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D17775