doc: Start converting readme's TODOs into GitHub issues

Filed issues zulip#161, zulip#162, zulip#163, zulip#164, zulip#165, zulip#166, zulip#167, zulip#168, zulip#169,
 zulip#170, zulip#171, zulip#172, zulip#173, zulip#174, zulip#175.

Author: gnprice

README.md

@@ -126,34 +126,17 @@ To update the version bounds:
 
 ## TODO
 
-### Server API
+In general, see [the issue tracker][].
+Some items written below in an early phase of the project
+may not yet have been converted into issues.
 
-Much more to write.
-
-
-### State and storage
-
-Much more to design and write.
+[the issue tracker]: https://github.com/zulip/zulip-flutter/issues
 
 
 ### Message content
 
-If necessary we could put the message list in a webview, like we do
-in React Native.  But the current plan is to handle it with Flutter
-widgets.
-
 - Lots of specific types of elements; see TODO comments
 
-- Specific types of elements that may inform architecture:
-  - List item indicators according to nesting level
-  - Layout interactions like `p+ul`
-  - Lightbox for image attachments
-  - TeX
-
-- Font
-
-- Polls
-
 - Survey lots of messages to find unhandled types of elements
 - Survey all public messages on chat.zulip.org
 - Survey all public messages on [listed open communities][]
@@ -163,17 +146,6 @@ widgets.
 
 ### Message list, other than content
 
-- Show more/better message metadata:
-  - Starred
-  - Edited/moved
-
-- Handle layout/UI interactions between messages:
-  - Sender names/avatars
-  - Recipient headers
-  - Date separators
-
-- UI to interact with messages
-
 - Scroll position at first unread, or via link (vs. latest)
 
 - Dark theme (and in content too)

lib/model/message_list.dart

@@ -98,8 +98,8 @@ class MessageListView extends ChangeNotifier {
 
   static Iterable<ZulipContent> _contentsOfMessages(Iterable<Message> messages) {
     // This will get more complicated to handle the ways that messages interact
-    // with the display of neighboring messages: sender headings,
-    // recipient headings, and date separators.
+    // with the display of neighboring messages: sender headings #175,
+    // recipient headings #174, and date separators #173.
     // TODO factor [messages] and [contents] into own class to encapsulate that
     return messages.map((message) => parseContent(message.content));
   }

lib/widgets/content.dart

@@ -145,12 +145,12 @@ class ListNodeWidget extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    // TODO p+ul and p+ol interactions
+    // TODO(#162): p+ul and p+ol interactions
     final items = List.generate(node.items.length, (index) {
       final item = node.items[index];
       String marker;
       switch (node.style) {
-        // TODO different unordered marker styles at different levels of nesting
+        // TODO(#161): different unordered marker styles at different levels of nesting
         //   see:
         //     https://html.spec.whatwg.org/multipage/rendering.html#lists
         //     https://www.w3.org/TR/css-counter-styles-3/#simple-symbolic