Rename getRowCount to getItemCount in SingleSelectionModel
Justification: When we first created SelectionModel we had it tightly aligned to the concept of 'rows', but this is incorrect: SelectionModel is being used in many places where 'row' makes no sense. For example, TabPane. A few months back we did a global API replace of references to row with references to index / item, depending on the context. This method was obviously missed, and should be changed to ensure both consistency and clarity when being used in controls that do not have a concept of rows.