Prevent unnecessary backpagination requests from running every time a room is entered

- related to #1760 which moved initial timeline item computations to a serial queue

ElementX/Sources/Screens/RoomScreen/RoomScreenModels.swift

@@ -215,7 +215,7 @@ struct RoomMemberState {
 /// Is also nice to have this as a wrapper for any state that is directly connected to the timeline.
 struct TimelineViewState {
     var canBackPaginate = true
-    var isBackPaginating = false
+    var isBackPaginating = true
     
     // These can be removed when we have full swiftUI and moved as @State values in the view
     var scrollToBottomPublisher = PassthroughSubject<Void, Never>()

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

@@ -46,10 +46,6 @@ struct UITimelineView: UIViewControllerRepresentable {
         
         init(viewModelContext: RoomScreenViewModel.Context) {
             context = viewModelContext
-            
-            if viewModelContext.viewState.timelineViewState.itemViewStates.isEmpty {
-                viewModelContext.send(viewAction: .paginateBackwards)
-            }
         }
         
         /// Updates the specified table view's properties from the current view state.

ElementX/Sources/Services/Timeline/TimelineController/RoomTimelineController.swift

@@ -57,8 +57,11 @@ class RoomTimelineController: RoomTimelineControllerProtocol {
             }
             .store(in: &cancellables)
         
+        // Inform the world that the initial items are loading from the store
+        callbacks.send(.isBackPaginating(true))
         serialDispatchQueue.async {
             self.updateTimelineItems()
+            self.callbacks.send(.isBackPaginating(false))
         }
         
         NotificationCenter.default.addObserver(self, selector: #selector(contentSizeCategoryDidChange), name: UIContentSizeCategory.didChangeNotification, object: nil)