React to config updates inside indexer

Summary:
In case a config was changed, we need to:
 1. Stop the ContentIndexer (bug 373430)
 2. Remove all scheduled indexing of files that should not be indexed
 3. Check for stale index entries. Those includes documents marked for indexing, that should be removed
(because of that we need to do it inside `scheduleIndexing` before content indexing)
 4. Finally, check for unindexed files (if new files were added)

(based on {D15939}). Also note that `IndexCleaner` should clean all files, not just those inside `includeFolders`

BUG: 373430

Test Plan:
Seems to be working. But it would be nice to test it more.

 1. Add large folder to config. `baloo_file` start indexing it.
 2. Add it to exclude list. `baloo_file` stops indexing it, and removes entries from DB.
 3. Remove it from exclude list. `baloo_file` starts indexing it again.

Reviewers: #baloo, #frameworks, bruns, ngraham

Reviewed By: #baloo, ngraham

Subscribers: kde-frameworks-devel, bruns

Tags: #frameworks, #baloo

Differential Revision: https://phabricator.kde.org/D15983