Don't crash on exit when a module attaches a menu item...

... such as Nyquist Workbench.

I don't fully understand why, but destroying the menu registry items very late,
during the destruction of static objects, causes a crash in memory deallocation,
at least on Mac.

So destroy the menu registry explicitly in application shut-down.

src/AudacityApp.cpp

@@ -2140,6 +2140,8 @@ int AudacityApp::OnExit()
 
    AudioIO::Deinit();
 
+   MenuTable::DestroyRegistry();
+
    // Terminate the PluginManager (must be done before deleting the locale)
    PluginManager::Get().Terminate();