[itsme]

Something like that could almost be done already: just create a Collection "Favorites" and add the songs to that collection.
A shortcut to do that without having to open the song editor is a great idea.
I'd like to generalize the request: shortcuts for "add to collection" / "remove from collection" would be great.
Maybe even more than one.
Collections that I would use that for would be "Favorites", "TODO", "Temporarily Transposed"