Apache FOP Code update (bug 41656)

Code update
by
AD
(fixes bug 41656)

:

Refactoring in the fo package:
-> removal of the childNodes instance member in fop.fo.FObj
-> addition of a firstChild instance member in fop.fo.FObj
-> addition of a siblings instance member in fop.fo.FONode
-> addition of a FONodeIterator interface in FONode + corresponding implementation in FObj
-> changed implementations of FObj.addChildNode(), .removeChild() and .getChildNodes()