feat!: fire MsgRead on subsequent MDNs

BREAKING CHANGE: previously this event only fired
if message state really did just transition
from `DC_STATE_OUT_DELIVERED` to `DC_STATE_OUT_MDN_RCVD`.
Now this is only the case for `MsgRead` events
that have the newly added `first_time == true`.

Closes https://github.com/deltachat/deltachat-desktop/issues/5220.

This is also useful for channels
as it facilitates updating the post (message) read count live.

Despite the fact that it's a breaking change,
this should not be problematic in most cases
because clients mostly use this event as an "it's time to reload"
indicator.

There is a case in Delta Chat Desktop where and adjustment
will be needed:
d1fbb30979/packages/frontend/src/stores/messagelist.ts (L119-L123)
It seems that the message state could later transition
to `DC_STATE_OUT_FAILED`, so we should not uncoditionally set it
to `DC_STATE_OUT_MDN_RCVD`.

Note that this does not expose `first_time` in CFFI.

src/mimeparser.rs

@@ -2596,8 +2596,12 @@ async fn handle_mdn(
             (msg_id, from_id, timestamp_sent),
         )
         .await?;
+    context.emit_event(EventType::MsgRead {
+        chat_id,
+        msg_id,
+        first_time: !has_mdns,
+    });
     if !has_mdns {
-        context.emit_event(EventType::MsgRead { chat_id, msg_id });
         // note(treefit): only matters if it is the last message in chat (but probably too expensive to check, debounce also solves it)
         chatlist_events::emit_chatlist_item_changed(context, chat_id);
     }