poll: Add UI feedback on poll delay.

We replaced the unawaited call to sendSubmessage() with an awaited call.
This ensures that the operation's start and completion can be tracked, which makes it possible to implement a loading indicator. With the previous unawaited behavior, there was no reliable way to measure the duration of the process.

Fixes: #1808

Author: MritunjayTiwari14

lib/widgets/poll.dart

@@ -49,13 +49,24 @@ class _PollWidgetState extends State<PollWidget> {
   }
 
   void _toggleVote(PollOption option) async {
+    final navigator = Navigator.of(context);
+
+    unawaited(showDialog(context: context,
+        barrierDismissible: false,
+        builder: (_) {
+        return Center(child: CircularProgressIndicator());
+      }
+    ));
+
     final store = PerAccountStoreWidget.of(context);
     final op = option.voters.contains(store.selfUserId)
       ? PollVoteOp.remove
       : PollVoteOp.add;
-    unawaited(sendSubmessage(store.connection, messageId: widget.messageId,
+    await(sendSubmessage(store.connection, messageId: widget.messageId,
       submessageType: SubmessageType.widget,
       content: PollVoteEventSubmessage(key: option.key, op: op)));
+
+    navigator.pop();
   }
 
   @override