Mark rooms as read on push and pop. (#462)

We don't have visibility into timeline item read status so can't do this continuously yet.

* Simplify RoomTimelineProvider's responsibilities.

ElementX/Sources/Screens/RoomScreen/View/RoomScreen.swift

@@ -29,6 +29,13 @@ struct RoomScreen: View {
             .overlay { loadingIndicator }
             .alert(item: $context.alertInfo) { $0.alert }
             .sheet(item: $context.debugInfo) { DebugScreen(info: $0) }
+            .task {
+                // Give a couple of seconds for items to load and to see them.
+                try? await Task.sleep(for: .seconds(2))
+                
+                guard !Task.isCancelled else { return }
+                context.send(viewAction: .markRoomAsRead)
+            }
     }
     
     var timeline: some View {