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

mu4e/mu4e-vars.el

@@ -435,7 +435,7 @@ I.e. a message with the draft flag set."
   '((t :inherit font-lock-preprocessor-face))
   "Face for the mark in the headers list."
   :group 'mu4e-faces)
- 
+
 (defface mu4e-header-key-face
   '((t :inherit message-header-name-face :bold t))
   "Face for a header key (such as \"Foo\" in \"Subject:\ Foo\")."
@@ -625,6 +625,11 @@ mu4e-compose-mode."
 	 :shortname "Sgn"
 	 :help "Check for the cryptographic signature"
 	 :sortable nil))
+     (:decryption .
+       ( :name "Decryption"
+	 :shortname "Dec"
+	 :help "Check the cryptographic decryption status"
+	 :sortable nil))
      (:size .
        ( :name "Size"
 	 :shortname "Size"