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/editor/src/display_map.rs

@@ -973,7 +973,7 @@ pub mod tests {
                                         position,
                                         height,
                                         disposition,
-                                        render: Arc::new(|_| Empty::new().boxed()),
+                                        render: Arc::new(|_| Empty::new().into_element()),
                                     }
                                 })
                                 .collect::<Vec<_>>();