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.
Imported primary keys also have unknown trust level, when they have
been generated in other keyrings.
Later, we should decide whether to exclude any primary key with ultimate
trust level from any further 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.
