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/AudioIO.h

@@ -800,6 +800,8 @@ class AUDACITY_DLL_API AudioIO final : public AudioIoCallback {
    AudioIO();
    ~AudioIO();
 
+   // This might return null during application startup or shutdown
+   static AudioIO *Get();
 
 public:
 
@@ -1189,4 +1191,6 @@ private:
    void StartStreamCleanup(bool bOnlyBuffers = false);
 };
 
+using AudioIOBase = AudioIO;
+
 #endif