Inserts an item into the LeadCollection at the specified index.
index
The 0-based index at which item should be inserted.
item
The object to insert. The value can be null.
Insert internally calls InsertItem, which may be overriden.
LeadCollection accepts null as a valid value and allows duplicate items.
If index is equal to Count, item is added to the end of LeadCollection.
This method raises the CollectionChanged event with Add action.