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/activity_indicator/src/activity_indicator.rs

@@ -343,20 +343,18 @@ impl View for ActivityIndicator {
                         .contained()
                         .with_margin_right(style.icon_spacing)
                         .aligned()
-                        .named("activity-icon")
+                        .into_named_element("activity-icon")
                 }))
                 .with_child(
                     Text::new(message, style.message.clone())
                         .with_soft_wrap(false)
-                        .aligned()
-                        .boxed(),
+                        .aligned(),
                 )
                 .constrained()
                 .with_height(style.height)
                 .contained()
                 .with_style(style.container)
                 .aligned()
-                .boxed()
         });
 
         if let Some(action) = action {
@@ -367,7 +365,7 @@ impl View for ActivityIndicator {
                 });
         }
 
-        element.boxed()
+        element.into_element()
     }
 }