Gets the index of the last node to be added to the selection.
In Multiselction mode then this property returns the index of the last node that was added to the selection, or -1 if nothing is selected.
When in single selection mode then this property returns the index of the currently selected node, or -1 if no node is selected.
This property can be set to -1 in both multis election and single selection mode to clear all selection.