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/deferred.rs

@@ -26,18 +26,18 @@ impl Deferred {
 }
 
 impl Element for Deferred {
-    type BeforeLayout = ();
-    type AfterLayout = ();
+    type RequestLayoutState = ();
+    type PrepaintState = ();
 
-    fn before_layout(&mut self, cx: &mut ElementContext) -> (LayoutId, ()) {
-        let layout_id = self.child.as_mut().unwrap().before_layout(cx);
+    fn request_layout(&mut self, cx: &mut ElementContext) -> (LayoutId, ()) {
+        let layout_id = self.child.as_mut().unwrap().request_layout(cx);
         (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,
     ) {
         let child = self.child.take().unwrap();
@@ -48,8 +48,8 @@ impl Element for Deferred {
     fn paint(
         &mut self,
         _bounds: Bounds<Pixels>,
-        _before_layout: &mut Self::BeforeLayout,
-        _after_layout: &mut Self::AfterLayout,
+        _request_layout: &mut Self::RequestLayoutState,
+        _prepaint: &mut Self::PrepaintState,
         _cx: &mut ElementContext,
     ) {
     }