Add a 'text_edited' submit trigger to 'Field' widget.

This trigger is useful if one wants to provide dynamic visual feedback
to the user as he types. For example, immediate color change in email
field as soon as the email becomes invalid.
Merge pull request #275 from agrawalprash/add-textedited-listener

Add a 'text_edited' submit trigger to 'Field' widget.  Looks good to me.