Remove unused e_cert_get_asn1_struct().
New base dependency: gcr-3 >= 3.4

Needed for GcrCertificateWidget and similar widgets.
ECert: Implement GcrCertificate interface.

ECert is now a type of GcrCertificate.
Use GcrCertificateWidget to view certificates.

e_cert_manager_new_certificate_viewer() now builds a simple GtkDialog
with a GcrCertificateWidget showing details about the given ECert.
Remove unused EASN1Object.
Remove unused e_cert_get_window_title().
Remove unused e_cert_get_issued_on_time().
Remove unused e_cert_get_expires_on_time().
Remove unused e_cert_get_issuers_chain().