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();
 

src/Menus.cpp

@@ -1059,6 +1059,11 @@ MenuTable::AttachedItem::AttachedItem(
    Registry::RegisterItem( sRegistry(), placement, std::move( pItem ) );
 }
 
+void MenuTable::DestroyRegistry()
+{
+   sRegistry().items.clear();
+}
+
 namespace {
 
 using namespace MenuTable;

src/commands/CommandManager.h

@@ -992,6 +992,8 @@ namespace MenuTable {
       {}
    };
 
+   void DestroyRegistry();
+
 }
 
 #endif