Delegate to track when some sort of batch change has occurred on this island that probably changes its connectivity. This is different from FOnGraphIslandNodeRemoved since FOnGraphIslandDestructiveChangeFinish will only be called once for the graph for a given operation while FOnGraphIslandNodeRemoved may be called multiple times if we're removing more than one node from the island at a given time. Note that this will only be called as a result of a destructive change. So repeatedly adding a node to an island won't call this event.