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/elements/constrained_box.rs

@@ -28,9 +28,9 @@ impl<V: View> ToJson for Constraint<V> {
 }
 
 impl<V: View> ConstrainedBox<V> {
-    pub fn new(child: Element<V>) -> Self {
+    pub fn new(child: impl Drawable<V>) -> Self {
         Self {
-            child,
+            child: child.into_element(),
             constraint: Constraint::Static(Default::default()),
         }
     }