A lot of fix because parentvalue has to be encoded to allow surveys to be multilang

changes to the db structure:
1) parentcontent is no longer the content required to the parent item but the corresponding encoded value, so I renamed it to parentvalue

More changes
- added $evaluateparentcontent to item class __construct to avoid using time to calculate useless parentvalues
- added item_encode_parentcontent method
- added item_decode_parentvalue method
- userform_set_prefill methods were changed according to parentvalue
- userform_save_preprocessing methods were changed according to parentvalue
- userform_get_parent_disabilitation_info methods were changed according to parentvalue