Prepare to split AudioIOBase from AudioIO...

... New files, but (almost) empty; don't use the global variable gAudioIO,
but use one of two accessor function names (which are the same function for
now).

AudioIOBase will have fewer dependencies than AudioIO -- in particular, no
dependency on tracks.

It won't include StartStream.  It will contain functions to query the
present state of streams, and device capabilities.

src/LabelTrack.cpp

@@ -1770,6 +1770,7 @@ bool LabelTrack::DoCaptureKey(wxKeyEvent & event)
          // If we're playing, don't capture if the selection is the same as the
          // playback region (this helps prevent label track creation from
          // stealing unmodified kbd. shortcuts)
+         auto gAudioIO = AudioIOBase::Get();
          if (pProj->GetAudioIOToken() > 0 &&
                gAudioIO->IsStreamActive(pProj->GetAudioIOToken()))
          {