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/menus/HelpMenus.cpp

@@ -251,7 +251,7 @@ void QuickFixDialog::OnFix(wxCommandEvent &event)
       // preference dialogs.
       if( Str == "/SnapTo" )
       {
-         pProject->AS_SetSnapTo( 0 );
+         ProjectManager::Get( *pProject ).AS_SetSnapTo( 0 );
       }
       else
       {