sound: Pass a GvcMixerControl to the stream volume sliders

Since 9d612ff1 this is required,
otherwise we never set up the GtkAdjustment and so the scales don't
display any values or widget to make adjustments, meaning they can't be

Also call `notify_volume_cb()` from `update_ranges()` to initialise the
adjustment, since when we set the mixer control we now might not have
set the value.

Closes #652
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