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/settings/src/settings_file.rs

@@ -95,7 +95,7 @@ mod tests {
         }
 
         fn render(&mut self, _: &mut ViewContext<Self>) -> Element<Self> {
-            Empty::new().boxed()
+            Empty::new().into_element()
         }
     }