Handle standard Cmd-{Y,Z} key combinations in Mac wxTextCtrl

Use them to undo and redo, as expected.

src/osx/textctrl_osx.cpp

@@ -342,7 +342,7 @@ void wxTextCtrl::OnDropFiles(wxDropFilesEvent& event)
 
 void wxTextCtrl::OnKeyDown(wxKeyEvent& event)
 {
-    if ( event.GetModifiers() == wxMOD_CONTROL )
+    if ( event.ControlDown() )
     {
         switch( event.GetKeyCode() )
         {
@@ -361,6 +361,18 @@ void wxTextCtrl::OnKeyDown(wxKeyEvent& event)
                 if ( CanCut() )
                     Cut() ;
                 return;
+            case 'Z':
+                if ( !event.ShiftDown() )
+                {
+                    if ( CanUndo() )
+                        Undo() ;
+                    return;
+                }
+                // else fall through to Redo
+            case 'Y':
+                if ( CanRedo() )
+                    Redo() ;
+                return;
             default:
                 break;
         }