Provide convenience instances of Margins where all sides are the same

[JohnMcGuinness]

Currently in order to create a margin where all sides are equal we use new Margin(Size.SMALL) if there is more than one place in an application where a small margin is required we create a new instance of a Margin with small size.

I propose that we create constants for these types of margins such as Margin.ZERO, Margin.SMALL and Margin.MEDIUM, etc and deprecate, the new Margin(Size) constructor. This constructor should be made private. It makes no sense recreating identical instances of these Margins all over the place.