Need a way to lazily get the url of a track

[knatten]

When playing local files, providing the url of a track as it's added to the queue makes sense. However, this is not always a good idea when streaming files, as streaming services typically give you a time limited url. For instance, Dropbox gives you a url that is only valid for four hours. This means the current scheme breaks down for instance if the playlist is longer than four hours, or if the user pauses playback for more than four hours and then resumes playing.

We probably need some sort of callback that will get the url of a track just before it's about to start playing. This of course needs to be optional, as most users will still use the current "url": "..." scheme.