PIM-2574: No popup saying there is unsaved changes

When a product form is submited with invalid values, the page in
reloaded with theses values (even the good one) and if we clik on back
to the grid there is no warning message we are leaving a form with
unsaved values
Trigger the change event on the root form
Merge pull request #1600 from akeneo/PIM-2574

PIM-2574: No popup saying there is unsaved changes