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