Added callback method for Nestable to easily find out what has changed by user.
This method is called when user made a single action and it passes two arguments:
id of moved item and id of the new parent
It is very useful when after every action we would like to make an AJAX request to server
and update hierarchical structure in the database
improved dropCallback method

now it contains set of details necessary to understand what user fas
done, e.g. from which group, to which group moved element, what was
additional data attributes of moved element, etc.