Pref attributes (issue #49):
+ Added xf:bind constraint for all cases.
Note however that validation of forms data on submission is now turned off, so the constraints give no effect.
When (if ever) validation is turned on, it should be verified how constraint rules work.

* Small refactoring to minimalize code duplication.
Pref attributes (issue #49):
Constraint and warning mesage not applied when policy is "forbidden" (the case of SubjectSet).
Summarising:
* pref checkboxes are controlled dynamically to avoid selecting more than one (more than one per role for AgentSet),
* warning message is shown when trying to submit form with incorrectly selected pref attributes,
* validation constraint rules are added to check correctness of pref attributes on submit; this is, however, inactive, because validation is turned off generally.