Separate notion of configured audio backend from Disobedience/playrtp

Separate notion of configured audio backend from Disobedience/playrtp
audio backend.  This allows Disobedience to show the volume widget iff
it'll work.  If RTP is enabled then it uses the default audio backend
(which is what playrtp will pick); if not then it assumes the server
is capable of setting the volume.

Currently this will still go wrong on a Mac without RTP.