Add Window opacity and per-pixel transparency for shaped windows

Desktop apps can make their windows translucent and non-rectangular. A Window gains an opacity setting that fades the whole window and its contents, and a way to make the window background transparent so custom window shapes are possible, pairing naturally with TitleBarVisible = False. Controls keep their own opaque backgrounds, so a window can be see-through behind its controls. Supported natively on Windows and macOS, and on Linux wherever a compositor is running. Blur-behind materials such as acrylic and Mica are not part of this feature.