Add new methods to position items in absolute positionning

Add new methods to position items in absolute positionning (https://github.com/layoutBox/FlexLayout#4-absolute-positioning):

• vertically(: CGFloat) / vertically(: FPercent):
  Controls the distance child’s top and bottom edges from the parent’s edges. Equal to top().bottom().
• horizontally(: CGFloat) / horizontally(: FPercent):
  Controls the distance child’s left and right edges from the parent’s edges. Equal to left().right().
• all(: CGFloat) / all(: FPercent):
  Controls the distance child’s edges from the parent’s edges. Equal to top().bottom().left().right().

Added by Kuluum in Pull Request #146