Add Decryption field

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
Revert "Do not handle signature parts after verification"

This reverts commit 6e9b9ad2d0c81dc0ad8f7a5c1b4981742d9fee22.

Unfortunately the reverted commit breaks the Signature field for
encrypted and, at the same time, signed messages.

TODO: details button in the Signatures field does not work for such
cases because the signature is encrypted.

Conflicts:
	lib/mu-msg-part.c
Fix signatures' detail box for encrypted messages

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.
Merge pull request #500 from zakkak/decryption_field

Add decryption field