[Baloo KCM] Expose "Index hidden" config setting

Summary:
Also inset both "Index file content" and "Index hidden" so it is more
obvious both are subordinate to the global "Enable indexing".

Reviewers: #baloo, #vdg, ngraham

Reviewed By: #baloo, #vdg, ngraham

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D28024