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/collab_ui/src/sharing_status_indicator.rs

@@ -52,11 +52,10 @@ impl View for SharingStatusIndicator {
                 .constrained()
                 .with_width(18.)
                 .aligned()
-                .boxed()
         })
         .on_click(MouseButton::Left, |_, _, cx| {
             cx.dispatch_action(ToggleScreenSharing);
         })
-        .boxed()
+        .into_element()
     }
 }