Switching the hook 'get_media_manager' to a more "directed" tuple-hook

By switching this to a tuple that includes the media type in the key
itself, this requires iteration and execution of functions that
"check" that they are the right type.

This commit sponsored by Greg Grossmeier.  Thanks buddy! :)
Documenting the media_manager fetching hook

This commit sponsored by Christian Corrodi.  Thank you!