Keep message selection active when every message is unselected

Unselecting the last message used to end the selection, closing it
without the user asking. Selection mode is now its own state: Forward
starts it and it only ends with the close button, once the messages are
forwarded, when the timeline changes or the feature is turned off. With
nothing selected the forward button stays visible but disabled.

ElementX/Sources/Screens/Timeline/TimelineViewModel.swift

@@ -1200,7 +1200,7 @@ extension TimelineViewModel {
             .sink { [weak self] isEnabled in
                 self?.state.messageSelection.isEnabled = isEnabled
                 if !isEnabled {
-                    self?.state.messageSelection.selectedEventIDs.removeAll()
+                    self?.state.messageSelection.end()
                 }
             }
             .store(in: &cancellables)
@@ -1218,6 +1218,7 @@ extension TimelineViewModel {
             return
         }
         
+        state.messageSelection.isActive = true
         state.messageSelection.selectedEventIDs.insert(eventID)
     }
     
@@ -1239,7 +1240,7 @@ extension TimelineViewModel {
         guard state.messageSelection.isActive else { return }
         
         if isSwitchingTimelines {
-            state.messageSelection.selectedEventIDs.removeAll()
+            state.messageSelection.end()
             return
         }
         
@@ -1249,7 +1250,7 @@ extension TimelineViewModel {
     
     /// Ends the selection, either from the close button or once the selected messages have been forwarded.
     func clearMessageSelection() {
-        state.messageSelection.selectedEventIDs.removeAll()
+        state.messageSelection.end()
     }
     
     /// Forwards the selection in timeline order, which is unrelated to the order the messages were selected in.