Corrected possible attributes for input elements

[NiedziolkaMichal]

This PR slightly reworks web-forms.rnc & web-forms2.rnc, correcting list of possible form attributes for each input type. It made usage of INPUT_ATTRIBUTES in Assertions.java obsolete and was removed. Now any invalid attribute in input element shows error like that:

I have also cleaned up both rnc files a little bit, moving all attributes to web-forms.rnc & connecting input types which were present in both files.

Fixes #1149

I think it would be better to make web-forms-input.rnc and move all rules regarding input types in there. In web-forms2.rnc there would be just few rules, so we could be move them to web-forms.rnc and delete the file.
We could also change naming of types regarding form attributes, because currently they are under:

1. shared-form.attrs
2. common-form.attrs
3. input.attrs

What do you think?