Programmatically scroll/jump to an element without knowing it's current position

[Dimibe]

Currently it's only possible to use the scroll controller to jump or scroll to a certain index. Since the elements are sorted according to their assigned groups, it's not known on which position in the list the items are displayed.

In order to provide a better way to programmatically jump/scroll to elements inside StickyGroupedList the scroll controller should be extended to allow scroll/jump without knowing the index of an element.

For this to work, a new parameter is added to the list view with the name elementIdentifier which is a function which returns the unique identifier for an element. The GroupedItemScrollController gets two new methods scrollToElement and jumpToElement which take a identifier instead of index as argument.

StickyGroupedList(
 elements: _myElements,
 elementIdentifier: (element) => element.contractId,
 [...]
);
 itemScrollController.jumpToElement(identifier: myElementIdentifier);

Connected issue: grouped_list issue #155