DOM L3 schema generation updated for April WD's

I've committed changes to build.xml, dom3-combine.xsl and dom-to-xsd.xsl to allow the automatic generation of dom3.xsd from the recent working drafts.  There are several instances where context-specific (local) element definitions were required.  Since there is not a mechanism for this with DTD's, it would be difficult to automate producing an equivalent DTD.  Unless there is a significant need for a dom3.dtd, I'd like to drop it.