[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/CatDealsCollectionView/Sample/LoadingNode.m

@@ -53,7 +53,7 @@ static CGFloat kFixedHeight = 200.0f;
     [spinner startAnimating];
     return spinner;
   }];
-  [_loadingSpinner.style setSizeWithCGSize:CGSizeMake(50, 50)];
+  _loadingSpinner.style.preferredSize = CGSizeMake(50, 50);
     
   // add it as a subnode, and we're done
   [self addSubnode:_loadingSpinner];