Add a decryption field of the form
Decryption: 2 part(s) decrypted 1 part(s) failed
Meaning that 2 encrypted mime parts where successfully decrypted and 1
part failed. Note that the number 2 refers to the number of
successfully decrypted mime parts and not the number of successfully
decrypted encryptes multiparts, i.e., if an encrypted multipart
contains 4 parts and decryption is successful the field will be
Decryption: 4 part(s) decrypted
TODO: Add details button listing the names and indexes of the
decrypted (or not) mime-parts