More std:: style for wxString...

... Remove all uses of the three-valued Cmp comparison member function, which
returns 0 for equality, except in one place.  Use operators == and != instead.

(C++20's spaceship operator hasn't landed here yet!)

src/commands/CommandManager.cpp

@@ -577,7 +577,7 @@ wxMenuBar * CommandManager::GetMenuBar(const wxString & sMenu) const
 {
    for (const auto &entry : mMenuBarList)
    {
-      if(!entry.name.Cmp(sMenu))
+      if(entry.name == sMenu)
          return entry.menubar;
    }