[xdata] Do not require FORM_TYPE field in FilledForm
The FillableForm API is the most convenient way of filling out a
form. Currently only forms with a FORM_TYPE can be filled, due to a
restriction in FillableForm. This makes filling out untyped forms very
cumbersome.
This commit removes the required FORM_TYPE when using FilledForm (and
therefor FillableForm). Some subclasses of FilledForm already check
that the correct FORM_TYPE is set using FilledForm#ensureFormType().
