Fix TopicEcho plugin message display (#322)

- Change binding of width property in delegate (see: https://stackoverflow.com/questions/63767669/parent-is-null-in-listview-delegate-after-upgrade-to-qt-5-15)
- Use scoped reference to model.display (see: https://forum.qt.io/topic/92085/using-qstringlistmodel-as-model-in-listview)

Signed-off-by: Rhys Mainwaring <rhys.mainwaring@me.com>

src/plugins/topic_echo/TopicEcho.qml

@@ -97,8 +97,8 @@ Rectangle {
         currentIndex: -1
 
         delegate: ItemDelegate {
-          width: parent.width
-          text: display
+          width: listView.width
+          text: model.display
         }
 
         model: TopicEchoMsgList