mu/mu4e: use :mailing-list as the mailing-list property

In queries you can use list: as before

This helps for issue #2246.

lib/message/mu-fields.hh

@@ -320,7 +320,7 @@ static constexpr std::array<Field, Field::id_size()>
 	    {
 		Field::Id::MailingList,
 		Field::Type::String,
-		"list", {},
+		"mailing-list", "list",
 		"Mailing list (List-Id:)",
 		"list:mu-discuss.example.com",
 		'v',

mu4e/mu4e-headers.el

@@ -567,7 +567,7 @@ found."
       ;; 'To', otherwise show From
       (:from-or-to (mu4e~headers-from-or-to msg))
       (:date (format-time-string mu4e-headers-date-format val))
-      (:mailing-list (mu4e~headers-mailing-list (plist-get msg :list)))
+      (:mailing-list (mu4e~headers-mailing-list val))
       (:human-date (propertize (mu4e~headers-human-date msg)
                                'help-echo (format-time-string
                                            mu4e-headers-long-date-format