This change should have no user-facing effect.  The one spot where we
have an `if (_disposed)` check in editMessage prevents a state update
and a rebuild from happening.  This only applies if the store is
disposed before the edit request fails, but the MessageListView with
the edited message should get rebuilt anyway (through onNewStore)
when that happens.

While we do create outbox messages, there are in no way user-visible
changes since the outbox messages don't end up in message list views.
We create skeletons for helpers needed from message list view, but don't
implement them yet, to make the diff smaller.

For testing, similar to TypingNotifier.debugEnable, we add
MessageStoreImpl.debugOutboxEnable for tests that do not intend to
cover outbox messages.

This implements the waitPeriodExpired -> waiting state transition.

GitHub discussion:
  zulip#1472 (comment)

For the topic-list page app bar, we leave out the icon "chevron_down.svg"
since it's related to a new design (zulip#1039) we haven't implemented yet.
This also why "TOPICS" is not aligned to the middle part of the app bar
on the message-list page.

We also leave out the new topic button and topic filtering, which are
out-of-scope for zulip#1158.

The topic-list implementation is quite similar to parts of inbox page
and message-list page.  Therefore, we structure the code to make
it easy to maintain in the future. Especially, this helps us
(a) when we're changing one, apply the same change to the other,
where appropriate, and (b) later reconcile the differences they
do have and then refactor to unify them.

Figma design:
  https://www.figma.com/design/1JTNtYo9memgW7vV6d0ygq/Zulip-Mobile?node-id=6819-35869&m=dev

The "TOPICS" icon on message-list page in a topic narrow is a UX
change from the design.  See CZO discussion:
  https://chat.zulip.org/#narrow/channel/48-mobile/topic/Flutter.20beta.3A.20missing.20topic.20list/near/2177505

The icon was taken from CZO discussion:
  https://chat.zulip.org/#narrow/channel/530-mobile-design/topic/Topic.20list.20in.20channel/near/2140324

Fixes: zulip#1158

Co-authored-by: Zixuan James Li <[email protected]>

This 11px whitespace can be traced back to 311d4d5 in 2022.

While this is not present in the Figma design, it would be a
good idea to refine it in the future.  See discussion:
  zulip#1453 (comment)

[chris: expanded commit-message summary line]

Co-authored-by: Chris Bobbe <[email protected]>

Also removed a stale comment that refers to resolved issues
(zulip#173 and zulip#175).

We will reuse this helper when processing items for outbox messages.

Normally, instead of doing it manually, we would add new languages
from Weblate's UI.  In this case, the ones added in this commit are
not offered there.  So we will commit this to GitHub first, and let
Weblate pull the changes from there.

The language identifier zh is left empty, and is set to be ignored
on Weblate for translations.  Translator will be expected translate
strings for zh-Hans-CN and zh-Hant-TW instead.  We can add more
locales with the zh language code if users request them.

See CZO discussion on how we picked these language identifiers:
  https://chat.zulip.org/#narrow/channel/58-translation/topic/zh_*.20in.20Weblate/near/2177452

The Figma:
  https://www.figma.com/design/1JTNtYo9memgW7vV6d0ygq/Zulip-Mobile?node-id=6089-28394&m=dev

And while we're adding a test for it, also check that the chosen
channel icon is the intended one.

[chris: added tests]

Co-authored-by: Chris Bobbe <[email protected]>

[chris: removed unused import]

Co-authored-by: Chris Bobbe <[email protected]>

[chris: small formatting/naming changes]

Co-authored-by: Chris Bobbe <[email protected]>

Helpers for starting an edit interaction and dealing with the
confirmation dialog will be useful as we support retrieving
messages not sent.

This migrates the Polish and Russian translations (the only existing
non-en locales to translate this) to use the new string as well.

To prepare for adjusting one of the cases from 4px to 2px:
  zulip#1535 (comment)

This is where the progress bar for outbox messages will go, so this
is for consistency with that. Discussion:
  zulip#1453 (comment)

[chris: fixed to maintain 4px bottom padding in the common case
where the progress bar is absent]

Co-authored-by: Chris Bobbe <[email protected]>

[greg: cherry-picked from zulip#1379]

This will enable our translation contributors to start translating
these ahead of the upcoming launch, even though the features that
use them aren't yet merged to main.