Read the number of selected messages to VoiceOver

The count was only on the forward button at the bottom, so VoiceOver
users had to go and find it after every tap. While selecting, each
selectable message now starts its label with "N messages selected".
The label changes on every toggle, so VoiceOver reads the new count
followed by the message, as in other messaging apps.

ElementX/Sources/Screens/Timeline/View/Style/TimelineItemAccessibilityModifier.swift

@@ -13,7 +13,7 @@ enum TimelineItemAccessibilityMessageSelection {
     /// No selection is active, the item offers its regular message actions.
     case none
     /// A selection is active, the item acts as a toggle when it can be selected.
-    case selecting(isSelected: Bool)
+    case selecting(isSelected: Bool, selectedCount: Int)
 }
 
 private struct TimelineItemAccessibilityModifier: ViewModifier {
@@ -76,9 +76,13 @@ private struct TimelineItemAccessibilityModifier: ViewModifier {
                         action()
                     }
                 }
-        case .selecting(let isSelected):
+        case .selecting(let isSelected, let selectedCount):
             if let item = timelineItem as? EventBasedTimelineItemProtocol, item.isForwardable {
                 view
+                    .accessibilityLabel { label in
+                        Text(L10n.screenRoomSelectionCount(selectedCount))
+                        label
+                    }
                     .accessibilityAddTraits(isSelected ? [.isButton, .isSelected] : .isButton)
                     .accessibilityAction(.default, action)
             } else {

ElementX/Sources/Screens/Timeline/View/Style/TimelineItemBubbledStylerView.swift

@@ -50,6 +50,11 @@ struct TimelineItemBubbledStylerView<Content: View>: View {
         context.viewState.messageSelection.contains(timelineItem.id.eventID)
     }
     
+    private var accessibilitySelection: TimelineItemAccessibilityMessageSelection {
+        guard isMessageSelectionActive else { return .none }
+        return .selecting(isSelected: isSelected, selectedCount: context.viewState.messageSelection.count)
+    }
+    
     /// The base padding applied to bubbles on either side.
     ///
     /// **Note:** This is on top of the insets applied to the cells by the table view.
@@ -171,7 +176,7 @@ struct TimelineItemBubbledStylerView<Content: View>: View {
         // Figma overlaps reactions by 3
         VStack(alignment: alignment, spacing: -3) {
             messageBubbleWithActions
-                .timelineItemAccessibility(timelineItem, selection: isMessageSelectionActive ? .selecting(isSelected: isSelected) : .none) {
+                .timelineItemAccessibility(timelineItem, selection: accessibilitySelection) {
                     if isMessageSelectionActive {
                         toggleSelection()
                     } else {