Micro API Changes to fix valadoc generation

* While generating valadoc.org documentation, errors where reported.
  In order to fix them, some changes were introduced, like API changes

* GXml.last_error is a legacy field maked public for some reason,
  in 0.10.1 this is replaced by GXml.get_last_error() and GXml.set_last_error()
  in order to avoid fields in namespaces.