* mu4e: support the :size field in headers, message view

mu4e/mu4e-vars.el

@@ -462,7 +462,12 @@ headers)."
        ( :name "Signature"
 	 :shortname "Sgn"
 	 :help "Check for the cryptographic signature"
-	 :sortable nil))
+	 :sortable nil))   
+     (:size .
+       ( :name "Size"
+	 :shortname "Size"
+	 :help "Size of the message"
+	 :sortable t))
      (:subject .
        ( :name "Subject"
 	 :shortname "Subject"

mu4e/mu4e-view.el

@@ -182,9 +182,7 @@ plist."
 		(if datestr (mu4e~view-construct-header field datestr) "")))
 	    ;; size
 	    (:size
-	      (let* (size (mu4e-view-size msg)
-		      (sizestr (when size (format "%d bytes" size))))
-		(if sizestr (mu4e~view-construct-header field sizestr))))
+	      (mu4e~view-construct-header field (mu4e-display-size fieldval)))
 	    ;; attachments
 	    (:attachments (mu4e~view-construct-attachments-header msg))
 	    ;; pgp-signatures

mu4e/mu4e.texi

@@ -533,7 +533,8 @@ Some notes to explain what you see in the example:
 
 @itemize
 @item The fields shown in the headers view can be influenced by customizing
-the variable @code{mu4e-headers-fields}.
+the variable @var{mu4e-headers-fields}; see @var{mu4e-header-info} for the
+list of available fields.
 @item Instead of showing the @t{From:} and @t{To:} fields separately, you
 can use From/To (@t{:from-or-to} in @code{mu4e-headers-fields} as a more
 compact way to convey the most important information: it shows @t{From:}