Add scroll_area::State::velocity (#3300)

* Implement `has_momentum`

* Add doc comment

* Call it `has_velocity`

* Implement `velocity` method

---------

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

crates/egui/src/containers/scroll_area.rs

@@ -54,6 +54,11 @@ impl State {
     pub fn store(self, ctx: &Context, id: Id) {
         ctx.data_mut(|d| d.insert_persisted(id, self));
     }
+
+    /// Get the current kinetic scrolling velocity.
+    pub fn velocity(&self) -> Vec2 {
+        self.vel
+    }
 }
 
 pub struct ScrollAreaOutput<R> {