macos: add getter for NSWindow with macOS windowing

There may be various reasons that an application could need access to the underlying NSWindow that is being used to display the GdkMacosSurface contents. This provides a minimal API to do that without exposing our implementation details through public API.

As our rendering system is likely to change over time, we very much want to keep GdkMacosView, GdkMacosLayer, GdkMacosTile, and GdkMacosWindow all private implementation details which are subject to change.

As this is public API, we are a bit long-winded with the name so it is clear what is being accessed without polluting symbol names with things like "ns" as we used to.

This supercedes !4382 (closed) but also requires that the 4_8 macros land first.