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().
Now that FormWriter, with its write() method, is gone, there is no
reason the FormReader method should still be named read(). Renaming to
getField() as this is what DataForm also uses.
Apply builder pattern to form fields and replace getVariable() with
getFieldName(). Refer to the field name as "field name" instead of
"variable" everyone, just as XEP-0004 does.
Improve the high-level form API: introduce FilledForm and FillableForm
which perform stronger validation and consistency checks.
Also add FormFieldRegistry to enable processing of 'submit' forms
where the form field types are omitted.
Smack also now does omit the form field type declaration on 'submit'
type forms, as it is allowed by XEP-0004.
