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/gpui/examples/text.rs

@@ -31,7 +31,7 @@ impl gpui::View for TextView {
     }
 
     fn render(&mut self, _: &mut gpui::ViewContext<Self>) -> Element<TextView> {
-        TextElement.boxed()
+        TextElement.into_element()
     }
 }