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/events/payload.rs

@@ -171,14 +171,19 @@ pub enum EventType {
         msg_id: MsgId,
     },
 
-    /// A single message is read by the receiver. State changed from DC_STATE_OUT_DELIVERED to
-    /// DC_STATE_OUT_MDN_RCVD, see dc_msg_get_state().
+    /// A single message is read by a receiver.
     MsgRead {
         /// ID of the chat which the message belongs to.
         chat_id: ChatId,
 
         /// ID of the message that was read.
         msg_id: MsgId,
+
+        /// Read for the first time (e.g. by just one group member
+        /// / channel subscriber).
+        /// State changed from DC_STATE_OUT_DELIVERED to
+        /// DC_STATE_OUT_MDN_RCVD, see dc_msg_get_state().
+        first_time: bool,
     },
 
     /// A single message was deleted.