Add validation of signatures API to glib frontend
this merge request adds API for validation of signatures to glib frontend. It is based on André's work in issue #464 (I've asked him privately whether I can file it as a merge request and he agreed with that).
I've modified it to be more glib styled and compatible with current poppler.
It basically adds API to PopplerFormField via which you can access signature info for the field and tell whether the signature is valid or whether the used certificate is trusted. It also tells you how many form fields has signature in the document so we know that there are any.