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/egui_demo_lib/src/demo/misc_demo_window.rs

@@ -371,7 +371,7 @@ impl BoxPainting {
                 ui.painter().rect(
                     rect,
                     self.rounding,
-                    Color32::from_gray(64),
+                    ui.visuals().text_color().gamma_multiply(0.5),
                     Stroke::new(self.stroke_width, Color32::WHITE),
                 );
             }