Unwrapped support

Intended for @JsonUnwrapped support. In its current state it even works with attributes, but not with @JacksonXmlText.

I had to refactor a bit the class hierarchy to make this work properly, so this needs to be in 2.4. This **might** break backward compatibility if anything **outside** this module references `XmlBeanSerializer`; hopefully nobody does this because this class doesn't seem to be part of the public API.