Add support for a sentinel file to skip folder and its branches

[Optimal-Procedure885]

Would be great if you could add support for a sentinel file (ideally named by the user) whose presence in a folder means do not scan this folder or any of its children.

This would avoid SK scanning content in folders that may contain audio files, but are not part of a user’s library.

[paultaylor]

So DeleteDuplicates has an option to delete folders if they only contain certain files using glob syntax

and I plan to use a similar thing for ignoring files/folders when runing tasks like Fix Songs, detailed briefly in this issue