Move ProjectWindowBase to new files...

... So that you can use GetProject without other linkage dependencies

src/ProjectWindowBase.h

@@ -0,0 +1,37 @@
+/**********************************************************************
+
+Audacity: A Digital Audio Editor
+
+ProjectWindowBase.h
+
+Paul Licameli split from ProjectWindow.h
+
+**********************************************************************/
+
+#ifndef __AUDACITY_PROJECT_WINDOW_BASE__
+#define __AUDACITY_PROJECT_WINDOW_BASE__
+
+#include <wx/frame.h> // to inherit
+
+class AudacityProject;
+
+///\brief A top-level window associated with a project
+class ProjectWindowBase /* not final */ : public wxFrame
+{
+public:
+   explicit ProjectWindowBase(
+      wxWindow * parent, wxWindowID id,
+      const wxPoint & pos, const wxSize &size,
+      AudacityProject &project );
+
+   ~ProjectWindowBase() override;
+
+   AudacityProject &GetProject() { return mProject; }
+   const AudacityProject &GetProject() const { return mProject; }
+
+protected:
+   AudacityProject &mProject;
+};
+
+#endif
+