Fix back pagination (#432)

* Use pagination and start items for view state.

isBackPaginating and canBackPaginate are updated each time the timeline is rebuilt

* Update some timeline snapshots

The top section has gone, which has altered the layout slightly.

ElementX/Sources/Screens/RoomScreen/RoomScreenViewModel.swift

@@ -61,10 +61,14 @@ class RoomScreenViewModel: RoomScreenViewModelType, RoomScreenViewModelProtocol
                     }
                     
                     self.state.items[viewIndex] = timelineViewFactory.buildTimelineViewFor(timelineItem: timelineItem)
-                case .startedBackPaginating:
-                    self.state.isBackPaginating = true
-                case .finishedBackPaginating:
-                    self.state.isBackPaginating = false
+                case .canBackPaginate(let canBackPaginate):
+                    if self.state.canBackPaginate != canBackPaginate {
+                        self.state.canBackPaginate = canBackPaginate
+                    }
+                case .isBackPaginating(let isBackPaginating):
+                    if self.state.isBackPaginating != isBackPaginating {
+                        self.state.isBackPaginating = isBackPaginating
+                    }
                 }
             }
             .store(in: &cancellables)