Only allow one realtime effect at a time

Warning...this required a small hack that allowed updating the
menus even if the toplevel frame is not the project window.  This
is because with the active frame actually becomes the effect dialog
and UpdateMenus() bailed if it wasn't the project frame.

This should be removed when we get multiple effect support.

src/AudacityApp.h

@@ -87,6 +87,7 @@ enum
    HaveRecentFiles        = 0x02000000,
    IsNotSyncLockedFlag    = 0x04000000,  //awd
    IsSyncLockedFlag       = 0x08000000,  //awd
+   IsRealtimeNotActiveFlag= 0x10000000,  //lll
 
    NoFlagsSpecifed        = 0xffffffff
 };