feat: Add window-based constructor to macOS subclassing adapter

[mwcampbell]

This easily solves the issue we're having with #250, where we'd otherwise have to either write a little Objective-C or do some low-level stuff with the ObjC runtime from C code to make the SDL example compile on macOS. It might also help some Rust users, which is why I implemented the alternate constructor in the platform adapter itself.

[DataTriny]

Are you sure we won't need a similar function on the regular Adapter?

[mwcampbell]

I've chosen not to add a similar function on the regular adapter because the assumption is that if a user is using the regular (non-subclassing) adapter, then they're implementing their own NSView subclass already, which means either they're writing in Objective-C, or they already have a way of working with the ObjC runtime, such as the objc2 crate. So they already have a way of directly providing the view, probably because they instantiated it from their own class.