Only show clear-signed text with good signature in notepad

gpg outputs the clear-signed text also if the signature is bad or if it
couldn't be verified at all (e.g. because of missing signing keys). This
could mislead the user to think that they can trust the text. To avoid
this the notepad only shows the clear-signed text after signature
verification if a least one of the signatures (typically there is only
one) is good. Otherwise, the text in the notepad is cleared.

(cherry picked from commit 91e027afaf4980badc26b913b64687ad5786376e)

• GnuPG-bug-id: T8020