Instead of returning a usize for the window id, I'm instead returning a
`WindowHandle<V: View>` where `V` is the type of the window's root view.
@as-cii helped me with a cool technique using generic associated types
where methods on `WindowHandle` can return either T or Option<T>
depending on the `BorrowWindowContext::Result` associated type.
Some example usage...
```rs
let window = cx.add_window(|cx| MyView::new(cx));
let my_view = window.root(cx); // If cx is TestAppContext, returns MyView. Otherwise returns Option<MyView>, because the window could be closed.
```
This isn't insanely beneficial on its own, but I think it will help
clean up our testing story. I'm planning on making `window` more useful
in tests for laying out elements, etc.
- [x] Rework tests that call `add_window` 😅 to expect only a window in
return.
- [x] Get tests passing
- [x] 🚬 test
Nathan Sobo
d3c1966d96