Gvc is the actual "source of truth" when it comes to the currently active input
or output device, so we should update widget visibility and the input/output
meter stream based on the information from gvc, not on changes to the combo box.
The flow when the selects a different device in the combo box now is:
"user change" -> gvc_mixer_control_change_output() -> receive
"active-output-changed" signal from gvc -> update active device and get it
using the DeviceComboBox -> set widget visibility and update stream based
on this device.
This fixes a few bugs where the stream for the input/output meter wouldn't
get updated properly on active device changes.
bf6f72278e
