msglist [nfc]: Add TODO on date-separator line about dark theme

This gives a path forward for dark theme (#95), even with the open
question we commented about in the previous commit.

Author: chrisbobbe

lib/widgets/message_list.dart

@@ -524,6 +524,7 @@ class DateSeparator extends StatelessWidget {
   //   - opacity 0.15
   static const _line = BorderSide(
     width: 0,
+    // TODO(#95) in dark theme, use white, following web
     color: Colors.black);
 
   @override