Upgrade to CSS4J 0.31

Version 0.30
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 * Core:
- Initial CSS-enabled document DOM implementation.
- Rename method loadXHTMLDefaultSheet() to htmlDefaultSheet(). 
  loadXHTMLDefaultSheet() is now deprecated.
- Added method isVisitedURL(URL) to UserAgent. Can return false if the agent
  does not support history.
- Added method isVisitedURI(href) to CSSDocument.
- Added two convenience method overrides to CSSDocument.
- CSSComputedProperties now inherits from CSS2Properties.
- Added method getSelectorsForPropertyValue to style sheets.
- In parseCSSStyleSheet, SAC CSSExceptions are mapped to DOMExceptions,
  removing the former from method signature.
- Refactor to style sheets and factories. Includes the new interface 
  ExtendedCSSStyleSheet.
- Several classes in doc.style.css.dom changed to package visibility, and
  introduction of the ExtendedCSSFontFaceRule interface.
- Support for DocumentType objects in DefaultEntityResolver.
- A more appropriate implementation for isSameNode in DOM wrapper node 
  implementation.
- Support the identity unit conversion in CSSNumberValue (initial and final 
  units are the same).
- Updated package.info for the doc.style.css package.

 * Dom4j module:
- Adapt the dom4j backend to changes in core.

Version 0.31
------------
 * Core:
- DOM implementation: several changes for better efficiency and spec compliance.
- DOM implementation: enable live updates of style-related nodes, putting it on 
  par with the DOM4J backend.
- DOM implementation: remove support for entity references having child nodes.
- Two convenience Document method overrides for CSSDocument.
- Make getPropertyCSSValue(String) return a ExtendedCSSValue in 
  CSSComputedProperties.
- Return the specified value in getMinifiedCssText(String) for uri values.
- Move DOMStringListImpl to package io.sf.carte.doc.
- Added method parseCSSStyleSheet(InputSource, boolean) to 
  ExtendedCSSStyleSheet.
- Better implementation of CSSStyleDeclaration equals and hashCode.
- Micro refactor: push getText() implementation from DOMCSSStyleDeclaration to 
  ComputedCSSStyle.
- Adjustments to the equals, hashCode or clone methods of a few property value 
  classes.
- getCssText(): Try to represent colors in hexadecimal notation if they are in 
  range, instead of functional notation.
- getComputedStyle: if a color is represented by an identifier, try to give a 
  computed color value instead.
- Added the mergeState method to SheetErrorHandler.
- Minor correction of the javadoc wording for 
  CSSStyleSheetFactory.getErrorHandler().
- Code formatting changes in the doc.style.css package.

 * Dom4j module:
- Adapt to changes in core.
- XHTMLDocumentFactory: override a createDocument method variant and minor 
  changes.