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/src/test.rs

@@ -243,6 +243,6 @@ impl View for EmptyView {
     }
 
     fn render(&mut self, _: &mut ViewContext<Self>) -> Element<Self> {
-        Drawable::boxed(Empty::new())
+        Empty::new().into_element()
     }
 }