OpenUri() should work with empty tracklist

When mopidy is started clean, there is no tracklist or playlist and `OpenUri()` does not work at all.

These commits guard `OpenUri()` with `CanControl` instead of `CanPlay`. The [MPRIS2 specification](http://specifications.freedesktop.org/mpris-spec/latest/Player_Interface.html#Property:CanPlay) states that `CanPlay` only indicates if the `Play()` or `PlayPause()` methods can be used.