Fetch item class for each row of the data

[raviks789]

Currently, it is assumed that each list item in the list will be of same instance. Hence, calling getItemClass() method once in the assemble() method was sufficient to obtain the instance of the list item and build the entire list.

But if we want to build a list containing list items of different instances, we would have to obtain the instance for each list item. For instance, the root problem list for dependencies contains host, service or redundancy group. Hence, the list item can be an instance of HostListItem, ServiceListItem or RedundancyGroupListItem.

In which case, it would require us to rewrite parent assemble() method, which is not desirable. To avoid this createListItem() method must be introduced, that will be responsible for creating the list item.