extended form-fields autocorrection handling
- added functionality of per-form-field statistics
- added support for "radio"/single choice test items
- added support for richtext handling in "radio"
- added a (limited) method to set a form-field to a correct value, based on "correct_when".
So far, just a minimal set of predicated is handled
- improved "unknown" handling, when the code does not know, whether item is correct or not
- refactored some code
- improved source code documentation