Add new public method `addFields()` to HTMLForm

Add new public method `addFields()` to HTMLForm

Once you instantiate an HTMLForm there was no clear way of adding
new fields except for hidden fields. This is particularly problematic
when the form is passed by reference in Hooks.

NOTE: this is just moving what was previously part of the constructor
into its own method + very small tweaks

Change-Id: I23f983417510841ce76cdefcb076e5ab97b43f10