Disable Sign button of Notepad if invalid signing cert is selected

This prevents "Unusable secret key" errors if a certificate that cannot
be used for signing (e.g. expired, revoked, or otherwise invalid
signing subkey) is (pre-)selected as signing certificate.

(cherry picked from commit c9be9a0750d7a074740ea941af0ba07f21948f26)

• GnuPG-bug-id: T6788