RelativeSize.larger(), .smaller() (factory methods)
RelativeSize stores amount as Size internally
RelativeSize may be constructed from descriptors, not just numbers
ElementSize.plus()/minus() takes descriptor as parameter
INTEGRATE: ElementSize.plus()/minus() takes descriptor parameter

* dev:
  ElementSize.plus()/minus() takes descriptor as parameter
  RelativeSize may be constructed from descriptors, not just numbers
  RelativeSize stores amount as Size internally
  RelativeSize.larger(), .smaller() (factory methods)