New timeline (#276) (#280)

* Fixes #276 - Rebuilt room timeline:
    - Removed the need for the ListCollectionViewAdapter
    - Rewrote the TimelineItemList without using introspection
    - Added ReversedScrollView for laying out items at the bottom/trailing
    - Rewrote TimelineProvider diffing through CollectionDifference (similar to the RoomSummaryProvider)
    - Added back `scrollDismissesKeyboard`  behavior
    - Various other tweaks and fixes
- Fixed various warnings:
    - removed async AttributedStringBuilder as AttributedString is non-sendable, made the RoomTimelineItemFactory synchronous
    - removed unused virtual timeline items
    - removed unused isOutgoing property from the FormattedBodyText
* Make TimelineItemContextMenuActions indentifiable and specify contextMenu identifiers
* Bump the matrix-rust-components-swift to v1.0.16-alpha
* Add changes file and changelog contribution guide
* Fix attributed string builder unit tests

ElementX/Sources/Screens/RoomScreen/RoomScreenViewModel.swift

@@ -58,6 +58,10 @@ class RoomScreenViewModel: RoomScreenViewModelType, RoomScreenViewModelProtocol
                     }
                     
                     self.state.items[viewIndex] = timelineViewFactory.buildTimelineViewFor(timelineItem: timelineItem)
+                case .startedBackPaginating:
+                    self.state.isBackPaginating = true
+                case .finishedBackPaginating:
+                    self.state.isBackPaginating = false
                 }
             }
             .store(in: &cancellables)
@@ -81,11 +85,13 @@ class RoomScreenViewModel: RoomScreenViewModelType, RoomScreenViewModelProtocol
     override func process(viewAction: RoomScreenViewAction) async {
         switch viewAction {
         case .loadPreviousPage:
-            state.isBackPaginating = true
+            guard !state.isBackPaginating else {
+                return
+            }
             
             switch await timelineController.paginateBackwards(Constants.backPaginationPageSize) {
             default:
-                state.isBackPaginating = false
+                #warning("Treat errors")
             }
         case .itemAppeared(let id):
             await timelineController.processItemAppearance(id)