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/ViewInfo.cpp

@@ -14,7 +14,7 @@ Paul Licameli
 
 #include <algorithm>
 
-#include "AudioIO.h"
+#include "AudioIOBase.h"
 #include "prefs/GUISettings.h"
 #include "Prefs.h"
 #include "Project.h"
@@ -245,6 +245,7 @@ bool ViewInfo::ReadXMLAttribute(const wxChar *attr, const wxChar *value)
 
 void ViewInfo::OnTimer(wxCommandEvent &event)
 {
+   auto gAudioIO = AudioIOBase::Get();
    mRecentStreamTime = gAudioIO->GetStreamTime();
    event.Skip();
    // Propagate the message to other listeners bound to this