Changes in Field.py (log instead of transaction_note) and in browser/utils.py (always return string when using displayList)

Hi,

could you please review this and merge?

The first change in Field.py was discussed with @vangheem on iRC, using transaction_note to show that a new field is added on an existing object is not practical in some case because it shows to much informations in the undo_form, we better use a message in the zope log.

Second change is in browser/utils.py in the translate method that returns a tuple () if the value is empty and returns a string 'vaue 1, value 2' if the value was not empty.  This is a bit incoherent and we better return an empty string in the first case.

Please merge/review/discuss/shout/... ;-)

Gauthier