New attached structure ProjectManager handles project lifetime...

... that is, a factory function, open, close, import, undo/redo/rollback.

Also the callbacks from AudioIO, which need to invoke undo history push when
recording stops.

It is meant as a high-level class using several of the other things attached
to the project, while AudacityProject will be a low level class acting mostly
as just the container of the attached structures.

src/AutoRecovery.cpp

@@ -227,7 +227,7 @@ static bool RecoverAllProjects(AudacityProject** pproj)
       // Open project. When an auto-save file has been opened successfully,
       // the opened auto-save file is automatically deleted and a NEW one
       // is created.
-      AudacityProject::OpenProject( proj, files[i], false );
+      (void) ProjectManager::OpenProject( proj, files[i], false );
    }
 
    return true;