[mpris] Playlist extension

[Johannes Sasongko]

> It appears the API does not assume a playlist is a
> static list of songs to play, but could potentially be a dynamic playlist or
> an online service?
>
> Perhaps the type of the playlist should be revealed? I could imagine a
> applet using an icon to indicate it.

If that's the use case, perhaps it's better to simply expose an "icon
URL" property, rather than trying to anticipate every playlist type
that may exist?

Johannes