forms: fix isSubmitted

Implementation made wrong assumptions. A form is submitted when the
submit button has been pressed. It's value is then filled, it also
is when you're just pressing "RETURN". RETURN triggers the FIRST
submit button in a form. This way we are also able to find out which
form button has been pressed.

Current implementation is still poor, however isSubmitted works as
expected right now - and so does autosubmission.

fixes #5967