msglist: Handle update_message_flags events in model

After this, we'll be able to show a message's flags in the UI, such
as unread markers (#79).

Author: chrisbobbe

lib/model/message_list.dart

@@ -420,6 +420,35 @@ class MessageListView with ChangeNotifier, _MessageSequence {
     notifyListeners();
   }
 
+  void maybeUpdateMessageFlags(UpdateMessageFlagsEvent event) {
+    final isAdd = switch (event) {
+      UpdateMessageFlagsAddEvent()    => true,
+      UpdateMessageFlagsRemoveEvent() => false,
+    };
+
+    bool didUpdateAny = false;
+    if (event.all) {
+      for (final message in messages) {
+        (isAdd ? message.flags.add : message.flags.remove).call(event.flag);
+        didUpdateAny |= true;
+      }
+    } else {
+      for (final messageId in event.messages) {
+        final index = _findMessageWithId(messageId);
+        if (index != -1) {
+          final message = messages[index];
+          (isAdd ? message.flags.add : message.flags.remove).call(event.flag);
+          didUpdateAny |= true;
+        }
+      }
+    }
+    if (!didUpdateAny) {
+      return;
+    }
+
+    notifyListeners();
+  }
+
   void maybeUpdateMessageReactions(ReactionEvent event) {
     final index = _findMessageWithId(event.messageId);
     if (index == -1) {

lib/model/store.dart

@@ -311,7 +311,9 @@ class PerAccountStore extends ChangeNotifier {
       // TODO handle
     } else if (event is UpdateMessageFlagsEvent) {
       assert(debugLog("server event: update_message_flags/${event.op} ${event.flag.toJson()}"));
-      // TODO handle
+      for (final view in _messageListViews) {
+        view.maybeUpdateMessageFlags(event);
+      }
     } else if (event is ReactionEvent) {
       assert(debugLog("server event: reaction/${event.op}"));
       for (final view in _messageListViews) {