Merge pull request 'dont start 'new messages' when convo selected' (#453) from cache3.0 into trunk

Reviewed-on: https://git.openprivacy.ca/cwtch.im/cwtch-ui/pulls/453

lib/models/contact.dart

@@ -242,7 +242,9 @@ class ContactInfoState extends ChangeNotifier {
       unreadMessages++;
     }
     if (_newMarkerMsgIndex == -1) {
-      _newMarkerMsgIndex = 0;
+      if (!selectedConversation) {
+        _newMarkerMsgIndex = 0;
+      }
     } else {
       _newMarkerMsgIndex++;
     }