Fix inconsistency in custom fields position

Hello,

The following actions in form manager will cause an inconsistency in custom fields positions, two or more fields will share the same position :
 - Create some fields in a form (for having a sample)
 - Delete any field (but NOT the last one)
 - Add a new field

The last two fields will now share the same position (you can check in the database).


A fix like this one can resolve this bug, basically it decrease all the followings fields position before deleting the field.