poll [nfc]: Store key on PollOption

This allows us to lookup the key that corresponds to the PollOption
without referencing back the map.

Signed-off-by: Zixuan James Li <[email protected]>

Author: PIG208

lib/api/model/submessage.dart

@@ -462,7 +462,7 @@ class Poll extends ChangeNotifier {
 
     final key = PollEventSubmessage.optionKey(senderId: senderId, idx: idx);
     assert(!_options.containsKey(key));
-    _options[key] = PollOption(text: option);
+    _options[key] = PollOption(key: key, text: option);
     _existingOptionTexts.add(option);
   }
 
@@ -479,8 +479,9 @@ class Poll extends ChangeNotifier {
 }
 
 class PollOption {
-  PollOption({required this.text});
+  PollOption({required this.key, required this.text});
 
+  final PollOptionKey key;
   final String text;
   final Set<int> voters = {};