Introduce SettingState element to handle KCM's Items different states
handle immutability and indicate when a setting has been changed. Co-author Kevin Ottens
Task related : Figure out a good UI for the "show which settings have been changed" feature https://phabricator.kde.org/T13008
Part of the aforementioned feature, this patch introduces a SettingStateBinding element that ease the management of Items in the KCM when a KConfig setting associated is immutable or different from its default value.