Follow up of T8110.
* Add a hint in the warning box that, for a better feedback on what is encrypted/signed, the mail can be saved and then opened in kleopatra
* Rename attachments with different encryption/signature: "NOT_VERIFIED_<NAME>"/"UNGEPRUEFT_<NAME>"
* Adding a new tag "not verified attachments"/"Ungeprüfte Anhänge" for mails with one or more such attachments
* If possible and applicable, offer other mail parts als "<NAME>.eml" (instead of separate attachments test.txt and signature.asc, or Untilted attachment 00033.dat and msg.asc. This way the mail can be opened and decryption/verification just works as expected