gwtchosen-137 Disabled gwtchosen remains in tab index
 - Further improvements on tab index handling:
  - Introduced new event to change tab index. Event is fired if tab index is changed or the component gets enabled/disabled.
  - If component is disabled tab index is set to -1. If enabled the tab index of the original select element is used.
  - The tab index of the original select element is not changed anymore!