Disable the module if the setting is locked down

Summary:
In this case we simply disable the whole module since it manages only a
single key really. We could go finer grained allowing to add/remove
themes, but it'd be pointless really (and also removal has a side
effect on the selected theme anyway).

Reviewers: #plasma, mart

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D24699