Removed the DOM representation of the structure tree.

Removed the DOM representation of the structure tree.

The structure tree is now directly converted into corresponding PDF objects.
When going the IF route, the structure tree is stored in the form of SAX events that will be re-played when it's time to stream them into the output. This may still change.

* Extracted RFC3066 methods from XMLUtil into new LanguageTags class that can be re-used in a non-XML context.
* Dropped support for accessibility in the old Area Tree XML.
* Added support for the xml:lang property on fo:root, so that the global language can be set for the document without retrieving the language from the first page-sequence.
* Renamed StructureTreeBuilder into more appropriate StructureTreeEventHandler (same for applicable sub-classes)
* Renamed StructureTreeBuildingFOEventHandler into FO2StructureTreeConverter and added test case
* Added test cases for classes in the StructureTreeEventHandler hierarchy

git-svn-id: https://svn.apache.org/repos/asf/xmlgraphics/fop/branches/Temp_ImproveAccessibility@1205935 13f79535-47bb-0310-9956-ffa450edef68