Show the preferred resolution on the combo box

Summary: Usually there's a preferred resoluton for a display, let the user know it

Test Plan: Used the kcm with it, it's shown.

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: ngraham, davidedmundson, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D15776