Fix and extend verify dialog for CMS file signatures.
* src/gpastreamverifyop.c (done_cb): Factor sigdesc code out to ... * src/gpgmetools.c (gpa_gpgme_get_signature_desc): New. * src/verifydlg.c (fill_sig_model): Use new function. (add_signature_to_model): Simplify by using new function and fix bug with X.509 keys. (SignatureListColumn, signature_list): Add column SIG_DESC_COLUMN.