mu4e: allow sorting by list-id

Added support for sorting by mailing-list; note that this ultimately is
a sort by the 'list-id', so the items will be in that alphabetical
order, which is not necessarily the same as the order of the friendly
names.

mu4e/mu4e-headers.el

@@ -206,9 +206,9 @@ but also manually invoked searches."
   :group 'mu4e-headers)
 
 (defvar mu4e-headers-sort-field :date
-  "Field to sort the headers by.
-Field must be a symbol, one of: :date, :subject, :size, :prio,
-:from, :to.")
+  "Field to sort the headers by. Must be a symbol,
+one of: `:date', `:subject', `:size', `:prio', `:from', `:to.',
+`:list-id'")
 
 (defvar mu4e-headers-sort-direction 'descending
   "Direction to sort by; a symbol either `descending' (sorting
@@ -291,6 +291,7 @@ followed by the docid, followed by `mu4e~headers-docid-post'.")
 (defvar mu4e~headers-sort-field-choices
   '( ("date"	. :date)
      ("from"	. :from)
+     ("list"    . :list-id)
      ("maildir" . :maildir)
      ("prio"	. :prio)
      ("zsize"	. :size)

mu4e/mu4e-vars.el

@@ -745,11 +745,16 @@ mu4e-compose-mode."
 	 :shortname "Maildir"
 	 :help "Maildir for this message"
 	 :sortable t))
+     (:list .
+       ( :name "List-Id"
+	 :shortname "List"
+	 :help "Mailing list id for this message"
+	 :sortable t))
      (:mailing-list .
        ( :name "List"
 	 :shortname "List"
-	 :help "Mailing list for this message"
-	 :sortable nil))
+	 :help "Mailing list friendly name for this message"
+	 :sortable :list))
      (:message-id .
        ( :name "Message-Id"
 	 :shortname "MsgID"