Fix for bug #857

Even though this is only a P2, I felt it would be
quite bothersome for the users and might cause some
confusion.

The problem was the the wxLocale was being deleted
before all usage of it was complete.  The fix was
to explicitly delete the plugin and module managers.

src/AudacityApp.cpp

@@ -1908,6 +1908,10 @@ int AudacityApp::OnExit()
    // Terminate the PluginManager (must be done before deleting the locale)
    PluginManager::Get().Terminate();
 
+   // Done with plugins and modules
+   PluginManager::Destroy();
+   ModuleManager::Destroy();
+
    if (mLocale)
       delete mLocale;