closes #2332.  raises event when repopulating textbox

This is a very small change that enables JS widgets to be graded via hidden form fields.

The contract of supporting those on C2G is that the JS widgets update a hidden form field that captures its state (for autograding), and that the authors write a function that takes the text representation of the state and sets the JS widget appropriately.  That function needs to listen to the repopulate event on the hidden text field, and the platfrom needs to trigger the event on repopulate.  That's what this code does. (it's a 2 line change)