Floating scroll bars (#3539)

* Move scroll bar spacing settings to a `struct ScrollSpacing`

* Add a demo for changing scroll bar appearance

* Add setting for ScrollBarVisibility in demo

* Add `#[inline]` to a `ScrollArea` builder methods

* Refactor how scroll bar show/hide is computed

* Add support for floating scroll bars

* Tweak color and opacity of the scroll handle

* Allow allocating a fixed size even for floating scroll bars

* Add three pre-sets of scroll bars: solid, thin, floating

* Use floating scroll bars as the default

* Fix id-clash with bidir scroll areas

* Improve demo

* Fix doclink

* Remove reset button from demo

* Fix doclinks

* Fix visual artifact with thin rounded rectangles

* Fix doclink

* typos

crates/emath/src/vec2.rs

@@ -274,6 +274,16 @@ impl Vec2 {
         self.x.max(self.y)
     }
 
+    /// Swizzle the axes.
+    #[inline]
+    #[must_use]
+    pub fn yx(self) -> Vec2 {
+        Vec2 {
+            x: self.y,
+            y: self.x,
+        }
+    }
+
     #[must_use]
     #[inline]
     pub fn clamp(self, min: Self, max: Self) -> Self {