Introduce autoscroll support for elements (#10889)

This pull request introduces the new
`ElementContext::request_autoscroll(bounds)` and
`ElementContext::take_autoscroll()` methods in GPUI. These new APIs
enable container elements such as `List` to change their scroll position
if one of their children requested an autoscroll. We plan to use this in
the revamped assistant.

As a drive-by, we also:

- Renamed `Element::before_layout` to `Element::request_layout`
- Renamed `Element::after_layout` to `Element::prepaint`
- Introduced a new `List::splice_focusable` method to splice focusable
elements into the list, which enables rendering offscreen elements that
are focused.

Release Notes:

- N/A

---------

Co-authored-by: Nathan <nathan@zed.dev>

crates/gpui/src/elements/animation.rs

@@ -85,14 +85,14 @@ struct AnimationState {
 }
 
 impl<E: IntoElement + 'static> Element for AnimationElement<E> {
-    type BeforeLayout = AnyElement;
+    type RequestLayoutState = AnyElement;
 
-    type AfterLayout = ();
+    type PrepaintState = ();
 
-    fn before_layout(
+    fn request_layout(
         &mut self,
         cx: &mut crate::ElementContext,
-    ) -> (crate::LayoutId, Self::BeforeLayout) {
+    ) -> (crate::LayoutId, Self::RequestLayoutState) {
         cx.with_element_state(Some(self.id.clone()), |state, cx| {
             let state = state.unwrap().unwrap_or_else(|| AnimationState {
                 start: Instant::now(),
@@ -130,24 +130,24 @@ impl<E: IntoElement + 'static> Element for AnimationElement<E> {
                 })
             }
 
-            ((element.before_layout(cx), element), Some(state))
+            ((element.request_layout(cx), element), Some(state))
         })
     }
 
-    fn after_layout(
+    fn prepaint(
         &mut self,
         _bounds: crate::Bounds<crate::Pixels>,
-        element: &mut Self::BeforeLayout,
+        element: &mut Self::RequestLayoutState,
         cx: &mut crate::ElementContext,
-    ) -> Self::AfterLayout {
-        element.after_layout(cx);
+    ) -> Self::PrepaintState {
+        element.prepaint(cx);
     }
 
     fn paint(
         &mut self,
         _bounds: crate::Bounds<crate::Pixels>,
-        element: &mut Self::BeforeLayout,
-        _: &mut Self::AfterLayout,
+        element: &mut Self::RequestLayoutState,
+        _: &mut Self::PrepaintState,
         cx: &mut crate::ElementContext,
     ) {
         element.paint(cx);