msglist [nfc]: Add TODO(design) with unanswered question for date separator

In #469 implementing date separators, Greg highlighted that the date
separator's deviation from web wasn't settled yet:
  #469 (comment)
  #469 (comment)

Author: chrisbobbe

lib/widgets/message_list.dart

@@ -519,6 +519,13 @@ class DateSeparator extends StatelessWidget {
 
   final Message message;
 
+  // TODO(design) Do we want to match web more closely?
+  //   - width of 1 logical pixel instead of 1 physical pixel
+  //   - opacity 0.15
+  static const _line = BorderSide(
+    width: 0,
+    color: Colors.black);
+
   @override
   Widget build(BuildContext context) {
     // This makes the small-caps text vertically centered,
@@ -534,9 +541,7 @@ class DateSeparator extends StatelessWidget {
               child: DecoratedBox(
                 decoration: BoxDecoration(
                   border: Border(
-                    bottom: BorderSide(
-                      width: 0,
-                      color: Colors.black)))))),
+                    bottom: _line))))),
           Padding(padding: const EdgeInsets.fromLTRB(2, 0, 2, textBottomPadding),
             child: DateText(
               fontSize: 16,
@@ -546,9 +551,7 @@ class DateSeparator extends StatelessWidget {
             child: DecoratedBox(
               decoration: BoxDecoration(
                 border: Border(
-                  bottom: BorderSide(
-                    width: 0,
-                    color: Colors.black)))))
+                  bottom: _line)))),
         ])),
     );
   }