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/TrackPanel.cpp

@@ -471,6 +471,7 @@ void TrackPanel::OnTimer(wxTimerEvent& )
    auto &window = ProjectWindow::Get( *p );
 
    auto &projectAudioIO = ProjectAudioIO::Get( *p );
+   auto gAudioIO = AudioIO::Get();
 
    // Check whether we were playing or recording, but the stream has stopped.
    if (projectAudioIO.GetAudioIOToken()>0 && !IsAudioActive())