Signed 0.3.0

Seal signed copies with an optional digital certificate, thanks to @parnoldx.

When you add a signature, Digital seal is off by default. Turn it on by creating a certificate for that signature, picking one you already have, or importing a .p12 file. Copies saved with a sealed signature carry an invisible PKCS#7 signature over the whole document, so any later change to the file shows in every PDF viewer.

• Certificates are self-signed: a seal proves the file wasn't changed after you saved it, not who signed it, and some viewers warn that the signer is unknown.
• They live in ~/.local/share/signed/certificates/, which isn't created until you make or import one.
• A .p12 password is asked once for the import and never stored.