Fix for issue 3601 (Control Panel: unable to modify schema objects or attributes)

The following changes allow the user to edit and modify attributes and objectclasses (those that are considered not part of the configuration or standard schema).

With these changes the user can modify any property of the objectclass and the attributes, references to the modified elements (for instance when they are renamed) are updated in the schema.

These modifications also allow to delete schema elements that are referenced elsewhere in the schema.  Before the changes for instance deleting an attribute included in the definition of an object class was not allowed.  With these changes, the user is informed of the existing dependencies, but he/she is allowed to proceed with the deletion of the attribute (the references will be automatically removed).