Desktop dialog buttons cannot be assigned default and cancel actions

Custom desktop dialogs provide no supported way to designate an accept button for Return/Enter and a cancel button for Escape. Developers should be able to configure these button roles in the Window Designer and in Objo code, and have the corresponding key invoke the button's normal Pressed action.

Add default and cancel roles to desktop Buttons while preserving the focused control's own key handling. Existing buttons should keep their current behaviour until a role is explicitly assigned. The application's button handler should remain responsible for validation, saving, cancelling and closing its dialog.