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/AudacityApp.cpp

@@ -1619,6 +1619,7 @@ void AudacityApp::OnKeyDown(wxKeyEvent &event)
       auto scrubbing = scrubber.HasMark();
       if (scrubbing)
          scrubber.Cancel();
+      auto gAudioIO = AudioIO::Get();
       if((token > 0 &&
                gAudioIO->IsAudioTokenActive(token) &&
                gAudioIO->GetNumCaptureChannels() == 0) ||