adapter.aasx: Add XML deserialization support

This branch is already quite old and I couldn't really test it, since there are not working example AASX files.

However, I'm confident that all remaining errors when loading the AASX Explorer sample files are either subject to incompatible XML parsing (and thus not related to the AASXReader) or errors in the example files (i.e. they are not compliant with the standard).

There are still issues with the handling of File objects (see #81 (closed)), but this is unrelated to adding XML support.