msglist [nfc]: Rearrange to follow normal ordering of class members

In particular this causes the handful of places where each field
of MessageListView needs to appear to all be next to each other.

Author: gnprice

lib/model/message_list.dart

@@ -418,13 +418,19 @@ bool _sameDay(DateTime date1, DateTime date2) {
 ///  * When the object will no longer be used, call [dispose] to free
 ///    resources on the [PerAccountStore].
 class MessageListView with ChangeNotifier, _MessageSequence {
-  MessageListView._({required this.store, required this.narrow});
-
   factory MessageListView.init(
       {required PerAccountStore store, required Narrow narrow}) {
-    final view = MessageListView._(store: store, narrow: narrow);
-    store.registerMessageList(view);
-    return view;
+    return MessageListView._(store: store, narrow: narrow)
+      .._register();
+  }
+
+  MessageListView._({required this.store, required this.narrow});
+
+  final PerAccountStore store;
+  Narrow narrow;
+
+  void _register() {
+    store.registerMessageList(this);
   }
 
   @override
@@ -433,9 +439,6 @@ class MessageListView with ChangeNotifier, _MessageSequence {
     super.dispose();
   }
 
-  final PerAccountStore store;
-  Narrow narrow;
-
   /// Whether [message] should actually appear in this message list,
   /// given that it does belong to the narrow.
   ///