Improve HTMLSubmitField return value

Improve HTMLSubmitField return value

- do not return anything when the button was not clicked
- return boolean true (instead of the button text) when it was clicked

Unbreaks submit fields which currently don't return anything so there is
no easy way to tell whether they have been clicked.

Change-Id: If4e0dfb6ee6674f0dace80a01850e2d0cbbdb47a