feat: Don't put text into post-message

[Hocuri]

Before this PR, when a user with current main sends a large message to a user with an old Delta Chat (before #7431), the text will be duplicated: One message will arrive with only the text, and one message with attachment+text.

This PR changes this - there will be one message with only the text, and one message with only the attachment.

If we want to guard against lost pre-messages, then we can revert this PR in a few months, though I'm not sure that's necessary - it's unlikely that the small pre-message gets lost but the big post-message gets through.

Edit: The disadvantage of this PR is that message reordering can lead to lost text: If the pre-message arrives after the post-message, then the text won't be added to the post-message. I'm merging it anyways for now, since message reordering is very rare.

[Simon-Laux]

I think it is a good temporary solution.

[iequidoo]

Then, if messages arrive reordered, we need to add the pre-message as a new text message, maybe quoting the message with attachment. Or add the text to the attachment message and mark all messages starting from it as fresh. Just dropping the text away when we've finally received it is a bug.

[iequidoo]

#7712 looks as a better solution to me

[r10s]

#7712 has the big disadvantage, that the "Auto-Download Messages" option becomes completely non-working, with no chance for the user to fix it. that would probably even require UI adaption

with this PR, or even without, things get better over time when your friends upgrade

for out-of-order: well, that is a cornercase, esp when compared to "Auto-Download Messages" not working.
also, we can re-add the text anytime again.

so, i also think, this PR is a good temporary compromise, not letting messagess in old clients apper too weird

[iequidoo]

I though that #7712 is a better solution because i had a device with 2.34 and it wasn't receiving big messages from main Core at all, it was showing empty text messages w/o any download button. But now i can't reproduce the problem, i see an empty message followed by another message with the "Download" button

[iequidoo]

it's unlikely that the small pre-message gets lost but the big post-message gets through.

It may not necessarily depend on the message size, you can imagine a buggy server that drops every 10th message, or delays every 10th message. So as a temporary solution this PR can work, but not as a permanent, otherwise we need to warn users that images may arrive w/o text, etc.

[iequidoo]

Maybe, if the pre-message is lost, at least add some placeholder text explaining that or set msgs.error so that the user knows about the problem?