Spreadsheet's `Composite Data Set Index` widget not working correctly in all cases

• Open can.ex2, Apply
• Close active view, and show data in spreadsheet view
• Now in python shell, do this and observer the changes to the Composite Data Set Index widget on the Display properties after each property change

a = GetDisplayProperties()
a.CompositeDataSetIndex = [ 0 ]  # wrong widget state
a.CompositeDataSetIndex = [ 1 ]  # wrong widget state
a.CompositeDataSetIndex = [ 2 ]  # correct
a.CompositeDataSetIndex = [ 2, 3 ]  # semi-correct. the state for **Element Blocks**
                      # is wrong (should be fully checked, not partially checked, see below)

pqSignalAdaptorCompositeTreeWidget needs to be refactored