Make `hWnd: HWND` public

Right now `View` has `internal var hWnd: HWND!`. And If I want to make some extension, like DirectX rendering view, I can't integrate it with this frmaework. If make it public, I can create view and use it as target for DirectX rendering.
Any other changes, like adding new view subclasses, might also need access to `hWnd` if I need to make changes using native windows code.