drop urls into playlists

[goto-bus-stop]

It'd be neat if, in addition to drag/dropping existing media items and search results between playlists, you could also drag youtube and soundcloud URLs. if you have soundcloud open in one tab and you find a great track, you can just drag the url into a üWave playlist instead of copy/pasting the url into the search box and dragging it from there.

[goto-bus-stop]

This is easy to do entirely client-side for youtube videos because you can derive the sourceID from the URL, but for SoundCloud it would be best with a server-side change so you can add songs by their URL through the API.

I think we could just have u-wave-source-soundcloud accept a url in the sourceID when adding a song, and have it return the actual numeric SoundCloud sourceID in its response.