Self-signed certificates shown as `Signer unknown` instead of `Self signed`

[u1735067]

When importing/opening/pasting self-signed certificates, signature is shown as Signer unknown instead of Self signed, even if the certificate is already in XCA database. Self signed is only shown when the certificate is generated in XCA (pasting the same certificate shows it as Signer unknown).

This issue seems related to the logic at https://github.com/chris2511/xca/blob/RELEASE.2.9.0/widgets/CertDetail.cpp#L150-L159

Example from https://x509.io/ (X.509 Certificate: Ed25519 (RFC 8410)):

-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----