Decryption field

Add a decryption field of the form
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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

Revert commit 6e9b9ad
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Unfortunately the reverted commit breaks the Signature field for
encrypted and, at the same time, signed messages.

Fix signatures' detail box for encrypted messages
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In the case of encrypted and signed messages the signature field's
details box did not work due to missing flags to the mu verify command.
This commit fixes this issue.