TODO:
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- introduce constants for options to avoid typos

TODO XML_Serializer:
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- find a way to detect references in the structure and add id/idref attributes
- serialize some other structures, especially XPath return values
- custom serialization of objects

TODO XML_Unserializer:
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- support for xml:space attribute
- set types for certain tags (like in patConfiguration)
- namespace support
- custom unserialization for objects (static unserialize() or fromXML() methods)
- unserialize references using id/idref