KCM KDED: Add immutability and fix default, reset, apply buttons

Summary:
- Reset and apply button are not activated anymore if we change twice state of an item going back to the current saved state
- Immutability disable all actions for an entry
- Default is activated if not all mutable entry are set to true

Reviewers: ervin, crossi, meven, broulik

Reviewed By: ervin

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D26784