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/LyricsWindow.cpp

@@ -11,7 +11,7 @@
 
 #include "LyricsWindow.h"
 #include "Lyrics.h"
-#include "AudioIO.h"
+#include "AudioIOBase.h"
 #include "Prefs.h" // for RTL_WORKAROUND
 #include "Project.h"
 #include "ProjectAudioIO.h"
@@ -155,6 +155,7 @@ void LyricsWindow::OnTimer(wxCommandEvent &event)
 {
    if (ProjectAudioIO::Get( *mProject ).IsAudioActive())
    {
+      auto gAudioIO = AudioIOBase::Get();
       GetLyricsPanel()->Update(gAudioIO->GetStreamTime());
    }
    else