macos: add getter for NSWindow with macOS windowing

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.