This property maker handles the calculations described in 5.3.2 which
involves the sizes of the corresponding margin-* properties and the
padding-* and border-*-width properties.
A maker which calculates the line-height property.
This property maker is special because line-height inherits the specified
value, instead of the computed value.
So when a line-height is create based on an attribute, the specified value
is stored in the property and in compute() the stored specified value of
the nearest specified is used to recalculate the line-height.
Thin wrapper around a HashMap to implement the property caching idiom
in which a new Property instance is created then tested against cached
instances created previously.