NamedNodeMap.vala: add NamedMap interface and the NamedAttrMap implementation;...

NamedNodeMap.vala: add NamedMap interface and the NamedAttrMap implementation; may change NamedMap to NamedNodeMap yet; this will replace the GLib.HashTable usage for GXml.Node.attributes, and will allow us to rip out all the GXml<>libxml2 syncing code that occurs, since our NamedAttrMap will update the underlying libxml2 structures immediately when changes occur