Allow to search for Spotify playlists by Spotify URI.

This allows you to enter Spotify URI like e.g. `spotify:user:spotify:playlist:2Qi8yAzfj1KavAhWz1gaem` and will return the list of all tracks in this playlist. For some reason though the first time you click "Search" button, almost all tracks are returned like `[loading] spotify:track:0iVmK9TXETx9mBAiDnuo7k`. When you click "Search" button the second time, everything is ok. No idea why it works like that.
Could get the list of URI schemes handled from `mopidy.uri_schemes` and make this more generic?