QTI 2.1: enhance compatibility with Onyx Editor

Enhance the following points:

• Add a fallback encoding to open the ZIP files of tests
• Discovery the editor of a XML file (assessmentItem or assessmentTest) by errors in it

The main issues are files with Umlaut that cannot be unzipped and XML tests files without any indication of the editor or the version of the editor which need to be guess with an other method.

This is the option:
qti21.import.encoding.fallback=