Specifies level of trust of the signature.
Level 1 means that the signed key is asserted to
be a valid trusted introducer, with the 2nd octet of the body
specifying the degree of trust. Level
n means that the signed key is
asserted to be trusted to issue level
n-1 trust signatures.
This property is mapped to TElPGPTrustSignatureSubpacket.Level for version 4 signatures.