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-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"