WIP inbox: Add "Inbox" page!

TODO (before merge?):

- tests
- sorting (of streams; and within a stream, of topics)
- icons:
    https://chat.zulip.org/#narrow/stream/243-mobile-team/topic/design.3A.20.23F117.20.22Inbox.22.20screen/near/1680637
  (that message, and the next)
- currently the whole stream row is tappable to collapse/uncollapse;
  should we limit that to just the area near the
  arrow_down/arrow_right, and have the rest of the row open a
  message list to the stream narrow?
- @-mention indicator next to stream/topic unread badge count
- helper widgets; code structure etc.?
- commit msg. with "fixes" line

Other TODOs to be done in later work are noted in the code.

lib/widgets/app.dart

@@ -8,6 +8,7 @@ import 'package:flutter_gen/gen_l10n/zulip_localizations.dart';
 import '../model/localizations.dart';
 import '../model/narrow.dart';
 import 'about_zulip.dart';
+import 'inbox.dart';
 import 'login.dart';
 import 'message_list.dart';
 import 'page.dart';
@@ -249,6 +250,11 @@ class HomePage extends StatelessWidget {
                 narrow: const AllMessagesNarrow())),
             child: const Text("All messages")),
           const SizedBox(height: 16),
+          ElevatedButton(
+            onPressed: () => Navigator.push(context,
+              InboxPage.buildRoute(context: context)),
+            child: const Text("Inbox")),
+          const SizedBox(height: 16),
           ElevatedButton(
             onPressed: () => Navigator.push(context,
               RecentDmConversationsPage.buildRoute(context: context)),