sink: volume handling rework, new flat volume logic

- We now implement a logic where the sink maintains two distinct
  volumes: the 'reference' volume which is shown to the users, and the
  'real' volume, which is configured to the hardware. The latter is
  configured to the max of all streams. Volume changes on sinks are
  propagated back to the streams proportional to the reference volume
  change. Volume changes on sink inputs are forwarded to the sink by
  'pushing' the volume if necessary.

  This renames the old 'virtual_volume' to 'real_volume'. The
  'reference_volume' is now the one exposed to users.

  By this logic the sink volume visible to the user, will always be the
  "upper" boundary for everything that is played. Saved/restored stream
  volumes are measured relative to this boundary, the factor here is
  always < 1.0.

- introduce accuracy for sink volumes, similar to the accuracy we
  already have for source volumes.

- other cleanups.

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