History: Tracker Field Visibility

!Tracker field visibility

!!Actual
the isHiden field means
*'not hidden': every body who can see the item can see the field. 
*'visible by admin only': only admin can see the field and its value. 
*'editable by admin': everybody who can see the item can see the field and its value, but only admin can edit their value. 
*'visible by creator and admin': the field and its value can be seen in tracker list only for admin and only for a new item or an item belonging to a user (see @@ for the definition of belonging)

!!Proposal
Keep the actual ones:
*'not hidden': every body who can see the item can see and edit the field.
*'visible by admin only': only admin can see and edit the field and its value.
*'editable by admin only and not hidden': everybody who can see the item can see the field and its value, but only admin can edit their value. 
*'visible by creator and admin only': the field and its value can be seen in tracker list only for admin and only for a new item or an item belonging to a user 

2 new ones have been added in svn
*'visible By' a list of groups: After all the previous conditions have been checked, the user default group must be in the list of these groups to see the field. A field that is seen but not editable appears in an input form.
*'editable by' a list of groups: After all the previous conditions have been checked, the user default group must be in the list of these groups to see and edit the field

a third one to be added:
*'Editable by creator and admin' 

TODO: replace user default group with all the groups