Replace width and height with Widget::size

widget/src/scrollable.rs

@@ -220,12 +220,11 @@ where
         tree.diff_children(std::slice::from_ref(&self.content));
     }
 
-    fn width(&self) -> Length {
-        self.width
-    }
-
-    fn height(&self) -> Length {
-        self.height
+    fn size(&self) -> Size<Length> {
+        Size {
+            width: self.width,
+            height: self.height,
+        }
     }
 
     fn layout(