Drag and Drop into Playlists

Please implement dragging and dropping of tracks into playlists. Look into the NSTableViewDataSource protocol. You may not have to implement additional methods into a custom NSTableView subclass, but you may want to anyway, if for example you want custom animation as the user drags the songs over a playlist. In any case, it gets us farther away from iTunes. For a start, the key method is currently https://developer.apple.com/documentation/appkit/nstableviewdatasource/1525370-tableview. If using CoreData, write the selected managed objects’ uriRepresentation onto the pasteboard.