Add delegates to show room upgrades into the timeline model.

The delegates are at the beginning for upgraded rooms and end for predecessors.

Closes: network/neochat#620 and network/neochat#619

src/timeline/EventDelegate.qml

@@ -41,6 +41,20 @@ DelegateChooser {
         delegate: LoadingDelegate {}
     }
 
+    DelegateChoice {
+        roleValue: DelegateType.Predecessor
+        delegate: PredecessorDelegate {
+            room: root.room
+        }
+    }
+
+    DelegateChoice {
+        roleValue: DelegateType.Successor
+        delegate: SuccessorDelegate {
+            room: root.room
+        }
+    }
+
     DelegateChoice {
         roleValue: DelegateType.TimelineEnd
         delegate: TimelineEndDelegate {