msglist: Add topic list button to app bar

Add topic list button to app bar actions in message list view. The button
appears in channel narrows and navigates to the topic list page when
pressed. Uses the topic icon and localized tooltip.

Fixes zulip#1158.

Author: E-m-i-n-e-n-c-e

lib/widgets/message_list.dart

@@ -24,6 +24,7 @@ import 'sticky_header.dart';
 import 'store.dart';
 import 'text.dart';
 import 'theme.dart';
+import 'topic_list.dart';
 
 /// Message-list styles that differ between light and dark themes.
 class MessageListTheme extends ThemeExtension<MessageListTheme> {
@@ -275,6 +276,13 @@ class _MessageListPageState extends State<MessageListPage> implements MessageLis
         onPressed: () => Navigator.push(context,
           MessageListPage.buildRoute(context: context,
             narrow: ChannelNarrow(streamId)))));
+    } else if (narrow case ChannelNarrow(:final streamId)) {
+      (actions ??= []).add(IconButton(
+        icon: const Icon(ZulipIcons.topic),
+        tooltip: zulipLocalizations.topicListButtonTooltip,
+        onPressed: () => Navigator.push(context,
+          TopicListPage.buildRoute(context: context, streamId: streamId, messageListView: model!)),
+      ));
     }
 
     // Insert a PageRoot here, to provide a context that can be used for