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/shadow.rs

@@ -19,8 +19,12 @@ impl Shadow {
         color: Color32::TRANSPARENT,
     };
 
+    pub const fn new(extrusion: f32, color: Color32) -> Self {
+        Self { extrusion, color }
+    }
+
     /// Tooltips, menus, …, for dark mode.
-    pub fn small_dark() -> Self {
+    pub const fn small_dark() -> Self {
         Self {
             extrusion: 16.0,
             color: Color32::from_black_alpha(96),
@@ -28,7 +32,7 @@ impl Shadow {
     }
 
     /// Tooltips, menus, …, for light mode.
-    pub fn small_light() -> Self {
+    pub const fn small_light() -> Self {
         Self {
             extrusion: 16.0,
             color: Color32::from_black_alpha(20),
@@ -36,7 +40,7 @@ impl Shadow {
     }
 
     /// Used for egui windows in dark mode.
-    pub fn big_dark() -> Self {
+    pub const fn big_dark() -> Self {
         Self {
             extrusion: 32.0,
             color: Color32::from_black_alpha(96),
@@ -44,7 +48,7 @@ impl Shadow {
     }
 
     /// Used for egui windows in light mode.
-    pub fn big_light() -> Self {
+    pub const fn big_light() -> Self {
         Self {
             extrusion: 32.0,
             color: Color32::from_black_alpha(16),

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,