Position.plus() accepts other Positions
Position.plus() no longer accepts numbers (only Positions)
ElementCenter.plus, ElementCenter.minus
Rename ElementPosition --> RelativePosition
INTEGRATE: ElementCenter.plus, ElementCenter.minus

* dev:
  Rename ElementPosition --> RelativePosition
  ElementCenter.plus, ElementCenter.minus
  Position.plus() no longer accepts numbers (only Positions)
  Position.plus() accepts other Positions