Add a config option for setting the filesystem watcher policy

The watcher policy can be used to disable the checking for outside changes made to the filesystem, an admin can use this option to get a performance increase (particular when using external storages) if he knows no outside changes will be made to the filesystem.

To test:
- set `'filesystem_check_changes' => 0` in config.php
- perform an outside change to the filesystem (rename, delete, create)
- check if the change is not detected by in the files app
- set `'filesystem_check_changes' => 1` and reload the files app
- changes should now be detected

cc @DeepDiver1975 @PVince81