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