With this, if DigitalSignaturesDialog sets a signature description, then
that becomes part of the signed content. This is backwards-compatible in
two ways:

1) When the description is empty, the output is not changed.

2) When parsing, the description is optional: we only require that if
its hash is mentioned in the signed content, then the string matches its
hash; but omitting both the string and its hash is OK.

Change-Id: I82db5beea16bd325558a86201dc902d35543ed0a