Getting raw window handle, NSWindow, or the like

To allow applications to do platform-specific things not directly supported by jwm, such as injecting an accessibility implementation (e.g. as supported by [AccessKit](https://github.com/AccessKit/accesskit)), jwm should allow the user to get the platform-specific window handle (e.g. HWND or NSWindow pointer). I think it would make sense to do this via methods on the platform-specific Window subclasses, such as `winGetHwnd` and `macGetNSWindow`.