[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

AsyncDisplayKit/ASVideoNode.mm

@@ -250,8 +250,7 @@ static NSString * const kRate = @"rate";
   }
   
   if (_playerNode) {
-    _playerNode.style.width = ASDimensionMake(calculatedSize.width);
-    _playerNode.style.height = ASDimensionMake(calculatedSize.height);
+    _playerNode.style.preferredSize = calculatedSize;
     [_playerNode layoutThatFits:ASSizeRangeMake(CGSizeZero, calculatedSize)];
   }