[Layout] Add helper properties for setting sizes via CGSize and ASRelativeSize (#2324)

* Remove set layout style size helper

* Update API to set and get a CGSize / ASRelativeSize from an ASLayoutElementStyle

- Expose ASRelativeSize type
- Add new helper properties to set a size / relative size to ASLayoutElementStyle
- Don't expose the size in ASLayoutElementStyle anymore and move into ASLayoutElementStylePrivate

* Update examples

* Update comments for size helpers

examples/VerticalWithinHorizontalScrolling/Sample/ViewController.m

@@ -77,7 +77,7 @@
   CGSize boundsSize = pagerNode.bounds.size;
   CGSize gradientRowSize = CGSizeMake(boundsSize.width, 100);
   GradientTableNode *node = [[GradientTableNode alloc] initWithElementSize:gradientRowSize];
-  [node.style setSizeWithCGSize:boundsSize];
+  node.style.preferredSize = boundsSize;
   node.pageNumber = index;
   return node;
 }