[DMN Designer] ContextGrid and DecisionTableGrid selectFirstCell() regression

(Programmatic) selection of the "first cell" in both the ContextGrid and DecisionTableGrid has regressed. Selection should ignore the "row number" column however changes adding explicit "Row number columns" for the foregoing grids that do not extend RowNumberColumn have caused the expected behaviour to break.

Acceptance criteria

1. Create new Decision
2. Set expression to either Context or DecisionTable
3. The whole row should not be selected, only one cell.
4. Check other expression types.