Upgrade to CSS4J 2.0.1

CSS4J CHANGES
=============

Version 2.0.1
-------------
 * Core:
- NSAC: javadocs: better alignment of what the interfaces document and what the
  reference implementation actually does.
- NSAC impl.: allow real rgb color components (e.g. 'rgb(23.6 44.1 132.8)').
- XML-DTD: new StackedEntityResolver allows to use a resolver, and if it fails 
  to resolve, then use a fallback resolver.
- DOM impl.: avoid potential NPE in HTMLElement.setIdAttributeNode.
- CSSOM: add the sign() and abs() functions. See CSSWG issue 4673.
- CSSOM: initial support for constants PI and E in expressions.
- CSSOM: if the argument of a trigonometric function is dimensionless, accept 
  it and assume radians.
- CSSOM: stricter checking of unit validity in calc() expressions.
- CSSOM: allow real numbers in rgb color components.
- CSSOM: avoid potential NPE computing fallbacks of attr() functions.
- CSSOM: avoid potential NPE while serializing the 'background' shorthand in 
  getPropertyValue.
- CSSOM: fix a bug computing padding in the Simple Box Model.
- CSSOM: avoid a few potential NPEs in the Simple Box Model.
- CSSOM: HWBColorValue's equals() method was making a wrong type check.
- CSSOM: avoid NPE in FontFeatureValuesRule.equals(Object) method.
- CSSOM: avoid NPE in BaseCSSStyleDeclaration.diff(BaseCSSStyleDeclaration).
- CSSOM: do not serialize the 'margin' and 'padding' shorthands if any of the 
  longhands is a PROXY (e.g. var()).
- CSSOM: be a bit stricter in catching errors in linear gradient color stops.
- CSSOM: do not assume that var() with a color fallback always represents a 
  color.
- CSSOM: remove a dead field from DefaultStyleDeclarationErrorHandler.
- CSSOM: remove a couple useless value assignments from BorderRadiusBuilder.
- CSSOM: remove a useless value assignment in BaseCSSStyleDeclaration.
- Tests: avoid an unlikely NPE in style rule test.
- Tests: a field was needlessly static in several tests.
- Tests: upgrade to MetroUI CSS v4.3.5.
- Tests: upgrade to fontawesome v5.12.1.
- Several Javadoc fixes and improvements.
- Bump copyright notices to year 2020.

 * Dom4j module:
- A couple javadocs were still documenting the old (1.0) behaviour in the UA 
  sheet.
- Bump copyright notices to year 2020.


 * Agent module:
- Update to latest public suffix list.
- Bump copyright notices to year 2020.

 * AWT module:
- Bump copyright notices to year 2020.