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/img.rs

@@ -229,11 +229,11 @@ impl Img {
 }
 
 impl Element for Img {
-    type BeforeLayout = ();
-    type AfterLayout = Option<Hitbox>;
+    type RequestLayoutState = ();
+    type PrepaintState = Option<Hitbox>;
 
-    fn before_layout(&mut self, cx: &mut ElementContext) -> (LayoutId, Self::BeforeLayout) {
-        let layout_id = self.interactivity.before_layout(cx, |mut style, cx| {
+    fn request_layout(&mut self, cx: &mut ElementContext) -> (LayoutId, Self::RequestLayoutState) {
+        let layout_id = self.interactivity.request_layout(cx, |mut style, cx| {
             if let Some(data) = self.source.data(cx) {
                 let image_size = data.size();
                 match (style.size.width, style.size.height) {
@@ -256,21 +256,21 @@ impl Element for Img {
         (layout_id, ())
     }
 
-    fn after_layout(
+    fn prepaint(
         &mut self,
         bounds: Bounds<Pixels>,
-        _before_layout: &mut Self::BeforeLayout,
+        _request_layout: &mut Self::RequestLayoutState,
         cx: &mut ElementContext,
     ) -> Option<Hitbox> {
         self.interactivity
-            .after_layout(bounds, bounds.size, cx, |_, _, hitbox, _| hitbox)
+            .prepaint(bounds, bounds.size, cx, |_, _, hitbox, _| hitbox)
     }
 
     fn paint(
         &mut self,
         bounds: Bounds<Pixels>,
-        _: &mut Self::BeforeLayout,
-        hitbox: &mut Self::AfterLayout,
+        _: &mut Self::RequestLayoutState,
+        hitbox: &mut Self::PrepaintState,
         cx: &mut ElementContext,
     ) {
         let source = self.source.clone();