Thread fetch more button

Changes threads so there is a button to fetch more events. Also adds a separator between the thread root and the rest of the events.

src/timeline/ThreadBodyComponent.qml

@@ -87,6 +87,7 @@ ColumnLayout {
             }
             onShowMessageMenu: root.showMessageMenu()
             onRemoveLinkPreview: index => threadRepeater.model.closeLinkPreview(index)
+            onFetchMoreEvents: threadRepeater.model.fetchMoreEvents(5)
         }
     }
 }