[kcms/baloo] Offer to delete index after indexing is disabled

Split out from !427 (merged)

When the user disables indexing, the KCM now offers to remove the saved index data. This may be useful for privacy or security purposes.

BUG: 414077 FIXED-IN:5.22

Migrated from https://phabricator.kde.org/D25280

cc @bruns @teams/vdg