Simplify events handling

Let's replace current events system with three kind of events:

• add - when new element is added (separate event per element)
• delete - when element is deleted (separate event per element)
• change - whenever some change occurred in set.
  • It can be called once per many simultaneous changes, e.g. once per 2 adds and 3 deletes if they where done one after another.
  • It also can be called when in summary no elements where added and deleted, but ordering of elements in set have changed (e.g. when having { 1, 3 } set, we do set.clear(); set.add(3); set.add(1);. no add, and delete events will be invoked, however change event will occur.
  • It should also be allowed (for performance reasons) to emit change when change not necessarily may have occurred (and compute of whether it indeed occurred is expensive).
  • This event doesn't suppress add and delete, it's supplementary to first two