kcm_baloofile: Add option to disable file content indexing

Summary:
Baloo supports "only basic indexing" since version 5.15, which
causes it to only store file names into the database:
https://community.kde.org/Baloo/Configuration

Test Plan: Ran "balooctl config show contentIndexing" after changing the option.

Reviewers: #plasma, mart

Reviewed By: mart

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D5351