Rename Drawable::boxed to into_element and make containers generic

Multi-element are now generic over any drawable child, which can be converted
into an element.

Co-Authored-By: Nathan Sobo <nathan@zed.dev>
Co-Authored-By: Max Brunsfeld <max@zed.dev>

crates/outline/src/outline.rs

@@ -220,6 +220,6 @@ impl PickerDelegate for OutlineViewDelegate {
             .with_padding_left(20. * outline_item.depth as f32)
             .contained()
             .with_style(style.container)
-            .boxed()
+            .into_element()
     }
 }