[kcmkwin/rules] Return absolute path of decoration color scheme

Summary:
We need to return the absolute path instead of base name so decoColorToCombo
can return the correct index.

Test Plan:
* Go to System Settings > Window Management > Window Rules;
* Create a new rule for Konsole to force the Oxygen color scheme,
  close the dialog, and click the Apply button;
* Open the rules dialog (you don't have to modify anything), click OK button, then Apply button.

Without this patch, Breeze color scheme will be forced after the
last step.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: graesslin, broulik, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D17655