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/theme_selector/src/theme_selector.rs

@@ -217,6 +217,6 @@ impl PickerDelegate for ThemeSelectorDelegate {
             .with_highlights(theme_match.positions.clone())
             .contained()
             .with_style(style.container)
-            .boxed()
+            .into_element()
     }
 }