[win] close menu if user clicks outside

See also here ("Important: If you implement a context menu, it's vital that you set your window to the foreground before calling TrackPopupMenu to ensure the menu closes properly."):
https://www.codeproject.com/Articles/4768/Basic-use-of-Shell-NotifyIcon-in-Win32

macOS version re-written with objc-runtime

Removed  '-x objective-c' as it is no longer needed

Add platform-specific examples and update readme

Added submenu