Fixes for issues introduced with fox:destination:

Fixes for issues introduced with fox:destination:
DestinationElementMapping masks ExtensionElementMapping. That caused the tests for widow/orphan extension on tables to fail.
Improved Block child validation code (will need to be done for all FOs where fox:destination should be valid. Currently works only as child of fo:block or above fo:page-sequence where no layout managers are constructed.
No LMMaker for fox:destination. Improved the handling there so adding the extension element to the list there isn't necessary.

git-svn-id: https://svn.apache.org/repos/asf/xmlgraphics/fop/trunk@525526 13f79535-47bb-0310-9956-ffa450edef68