Using a workaround that validates passphrase for a secret key.
With GnuPG 2.2.23 and GpgME 1.1.15, a secret key can be exported when the right
passphrase is provided. With a bad passphrase, application crashes.
See https://dev.gnupg.org/T5151
Application may validate a passphrase before invoking engine. Until it is
hopefully fixed in upstream and available in mainstream.
A private key should be editable to Unknown or Ultimate levels only.
A public key should be editable to all levels except Ultimate.
Undefined level is excluded for both public and private keys.
The public WTreeTableNode may list private keys if the user does not manage
these private keys. Inform of this state in the tool tip. The certification
trust level is excluded for any change.
In-place editing with a popup if user is allowed in app config file.
User must of course manage at least a private key. User identities of
target key may be selectively chosen. Optionally, certification may be
exportable and non-revocable.
