Moved Xom to its own library

[Daniel Espinosa <esodan gmail com>]

I've moved all Xom stuff to its own library.

I've leave Serializable and JsonSerializable as was before. I think Richard, you must recover the old ones and move them.

The really think you must leave them in GXml namespace or create a new library, because add new a nested name space will cause troubles with GObject Introspection.

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