Convenience const fn for Margin, Rounding and Shadow (#4080)

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.

crates/epaint/src/shape.rs

@@ -701,7 +701,7 @@ impl Rounding {
     };
 
     #[inline]
-    pub fn same(radius: f32) -> Self {
+    pub const fn same(radius: f32) -> Self {
         Self {
             nw: radius,
             ne: radius,