KCM Baloo: Migrate to KConfigXT and add immutability

Summary: File indexing and Content file indexing benefits from immutability. But exclude/include list don't benefits from it because the UI represent both with the same list

Reviewers: ervin, crossi, meven, Plasma

Subscribers: meven, crossi, ervin, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D26401