... and also make `center_x` and `center_y` set
`width` and `height` to `Length::Fill`, respectively.
This targets the most common use case when centering
things and removes a bunch of boilerplate as a result.
This allows us to introduce a platform-specific `Action` to both `iced_native`
and `iced_web` and remove the `Clipboard` from `Application::update` to maintain
purity.
Additionally, this should let us implement further actions to let users query
and modify the shell environment (e.g. window, clipboard, and more!)
