Add a test on what happens currently when apps call `markseen_msgs()` for not downloaded
messages. Such messages are marked as seen, but MDNs aren't sent for them. This is probably correct,
but when a message is downloaded, its state should be downgraded to `InNoticed` and when the user
really sees it, the app should issue `markseen_msgs()` again and that should trigger sending an
MDN. Currently when such a message is downloaded, it remains `InSeen`.
Otherwise backups exported from the current core and imported in versions < 1.144.0 have QR codes
not working. The breaking change which removed the column is
5a6efdff44.
Before file extensions were also limited to 32 chars, but extra chars in the beginning were just cut
off, e.g. "file.with_lots_of_characters_behind_point_and_double_ending.tar.gz" was considered to
have an extension "d_point_and_double_ending.tar.gz". Better to take only "tar.gz" then.
Also don't include whitespace-containing parts in extensions. File extensions generally don't
contain whitespaces.
There is already code below that emits
progress 0 or 1000 depending on whether
configuration succeeded or failed.
Before this change cancelling resulted
in progress 0 emitted,
immediately followed by progress 1000.
Before this change progress bar only started
when database is already transferred.
Database is usually the largest file
in the whole transfer, so the transfer appears
to be stuck for the sender.
With this change progress bar
starts for backup export
as soon as connection is received
and counts bytes transferred over the connection
using AsyncWrite wrapper.
Similarly for backup import,
AsyncRead wrapper counts the bytes
received and emits progress events.
Instead of treating NULL type error
as absence of the row,
handle NULL values with SQL.
Previously we sometimes
accidentally treated a single column
being NULL as the lack of the whole row.
The "Cannot establish guaranteed end-to-end encryption with ..." info
message can have lots of causes, and it happened twice to us now that it
took us some time to figure out which one it is.
So, include some more detail in the info message by simply adding the
non-translated error message in parantheses.
If we want to put in some more effort for nicer error messages, we
could:
- Introduce one new translated string "Cannot establish guaranteed
end-to-end encryption with …. Cause: %2$s" or similar (and remove the
old stock string)
- And/Or: Introduce new translated strings for all the possible errors
- And/Or: Maybe reword it in order to account better for the case that
the chat already is marked as g-e2ee, or use a different wording
(because if the chat is marked as g-e2ee then it might be nice to notify
the user that something may have gone wrong, but it's still working,
just that maybe the other side doesn't have us verified now)

Info messages are added
at the beginning of unpromoted group chats
("Others will only see this group after you sent a first message."),
may be created by WebXDC etc.
They are not sent outside
and have local Message-ID that
is not known to other recipients
so they should be skipped when constructing
In-Reply-To and References.
Without this fix IMAP loop may get stuck
trying to download non-existing message over and over
like this:
```
src/imap.rs:372: Logging into IMAP server with LOGIN.
src/imap.rs:388: Successfully logged into IMAP server
src/scheduler.rs:361: Failed to download message Msg#3467: Message Msg#3467 does not exist.
src/scheduler.rs:418: Failed fetch_idle: Failed to download messages: Message Msg#3467 does not exist
```
The whole download operation fails
due to attempt to set the state of non-existing message
to "failed". Now download of the message
will "succeed" if the message does not exist
and we don't try to set its state.