Use HSSUnit for specificity and add fractions for properties applied via isA
Use 1.0 for specificity of properties that derive from other properties

Such as inner/outer width/height and margins/paddings.
Fix wrong property name
Use "modifiers", to correctly handle property paths and specificity

When using property paths, values are applied to properties inside values. When
substituting them, we want to keep those modifiers on the new object.
Set the specificity when creating values from parser nodes
Fix parsing error when using more than one property path node
Adjust constness