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/Project.h

@@ -286,7 +286,9 @@ class AUDACITY_DLL_API AudacityProject:  public wxFrame,
    static void DeleteClipboard();
    static void DeleteAllProjectsDeleteLock();
 
-   void UpdateMenus();
+   // checkActive is a temporary hack that should be removed as soon as we
+   // get multiple effect preview working
+   void UpdateMenus(bool checkActive = true);
    void UpdatePrefs();
    void UpdatePrefsVariables();
    void RedrawProject(const bool bForceWaveTracks = false);