Convenience const fn for Margin, Rounding and Shadow

[0Qwel]

I often write constants at the top of my widget files, as a "config". I kept writing stuff like that :

const DEFAULT_INNER_MARGIN: Margin = Margin { left: 17., right: 17., top: 7., bottom: 7. };

So I prefixed constructors for Margin, Rounding and Shadow const. No code was changed.

I also added a Shadow::new() for similar reasons.