Created a validateChildNode() in FONode for better syntax checking in the

Created a validateChildNode() in FONode for better syntax checking in the
fo document:  e.g., preventing two fo:layout-master-set's from occurring within
an fo:root, requiring fo:page-sequence to follow the fo:l-m-s
(and fo:declarations, if present).  Currently just validates fo:root's children--validateChildNode() needs to be overridden in other FObj subclasses
to do the same for other formatting objects.  Comments most welcome.

git-svn-id: https://svn.apache.org/repos/asf/xmlgraphics/fop/trunk@197703 13f79535-47bb-0310-9956-ffa450edef68