kcm_baloofile: Add option to disable file content indexing
Baloo supports "only basic indexing" since version 5.15, which
causes it to only store file names into the database:
Test Plan: Ran "balooctl config show contentIndexing" after changing the option.
Reviewers: Plasma, mart
Reviewed By: mart
Differential Revision: https://phabricator.kde.org/D5351