[macOS] 'About' and 'Quit' menu items get disabled if an application closes one of its multiple windows

If a window in a multi-window Qt application is closed then the 'About' and 'Quit' menu items are disabled in the remaining open windows. This behavior seems to be related to the macOS native menubar and its treatment of 'About' and 'Quit' items.

Steps to reproduce:

• Run the sample attached.
• Click on the "Clone Window" button -> new window appears.
• Close the newly opened window.
• Check the menu bar of the remaining window.
  EXPECTED: 'About' and 'Quit' menu items are enabled.
  ACTUAL: 'About' and 'Quit' menu items are disabled.