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_extras/src/table.rs

@@ -366,9 +366,9 @@ impl<'a> TableBuilder<'a> {
     fn available_width(&self) -> f32 {
         self.ui.available_rect_before_wrap().width()
             - if self.scroll_options.vscroll {
-                self.ui.spacing().scroll_bar_inner_margin
-                    + self.ui.spacing().scroll_bar_width
-                    + self.ui.spacing().scroll_bar_outer_margin
+                self.ui.spacing().scroll.bar_inner_margin
+                    + self.ui.spacing().scroll.bar_width
+                    + self.ui.spacing().scroll.bar_outer_margin
             } else {
                 0.0
             }