[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