Mapping between test cases and widgets

- Add attribute "name" in Link

- Add mapping description between transitions from Interaction of a Link
  and concretes widgets from the UI. 
  Widgets of the UI are targeted with a String ID
  
- Add widget IDs in InteractionTranstion of the IAFlowGraph metamodel.
  This information allows to find concretes widgets of the UI from
  test cases.
  
- In concretes values generation, InteractionTransition-Widget mapping 
  take care about widget ID.