HTMLForm: Allow to set the primary flag for submit buttons

HTMLForm: Allow to set the primary flag for submit buttons

Currently, every submit button generated by HTMLForm itself (not via
the descriptor) doesn't have the primary flag and there is no (easy) way
to add it, although this generated submit button is most likely the primary
one.

Add a new function HTMLForm::setSubmitPrimary() to allow a form to set
add the primary flag to this submit button.

Change-Id: I3ce363f995389a87764edb586b4b64ab2b77b0db