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/DeviceManager.cpp

@@ -259,6 +259,7 @@ void DeviceManager::Rescan()
    if (m_inited) {
       // check to see if there is a stream open - can happen if monitoring,
       // but otherwise Rescan() should not be available to the user.
+      auto gAudioIO = AudioIO::Get();
       if (gAudioIO) {
          if (gAudioIO->IsMonitoring())
          {