Various options and actions should only be visible if allowed

[sgielen]

I.e. don't display a "move up" button if the user won't be allowed to move the track up, or don't display a "skip" button if the user won't be allowed to skip.

[bwesterb]

The problem is, that we do not know the policy of the server. The server should advertise its policy.

Similarly, if we want flexible methods of requesting (etc.), the server should advertise a lot more.