Support undecorated windows

Desktop windows are allowed to be rendered "undecorated", which means the system does not draw any window title, controls etc. This is commonly used because the app wants to do these decorations itself, often because it wants better integration with the rest of the application. To support this in Flutter we need a mechanism for windows to disable decorations.

This feature is already being used in real applications like the [Ubuntu App Center](https://github.com/ubuntu/app-center) which uses [Handy Window](https://github.com/ubuntu/handy_window.dart) to disable decorations and [Yaru](https://github.com/ubuntu/yaru.dart) to render the window controls.