If the Inbox is fetched before the Sentbox (as done currently), messages from the Sentbox will
correctly mingle with the Inbox messages in the end. So, this commit changes message ordering only
if we already have processed outgoing messages, e.g. if we just sent them in the chat as described
in #4621. Otherwise new incoming messages are displayed somewhere in the middle of the chat which
doesn't look usable.
Correctly handle messages with old timestamps for verified chats:
* They must not be sorted over a protection-changed info message
* If they change the protection, then they must not be sorted over existing other messages, because then the protection-changed info message would also be above these existing messages.
This PR fixes this:
1. Even seen messages can't be sorted into already-noticed messages anymore. **This also changes DC's behavior in the absence of verified 1:1 chats**. Before this PR, messages that are marked as seen when they are downloaded will always be sorted by their timestamp, even if it's very old.
2. protection-changed info messages are always sorted to the bottom.
**Edit:**
3. There is an exception to rule 1: Outgoing messages are still allowed to be sorted purely by their timestamp, and don't influence old messages. This is to the problem described at [*].
Together, these rules also make sure that the protection-changed info message is always right above the message causing the change.
[*] If we receive messages from two different folders, e.g. `Sent` and `Inbox`, then this will lead to wrong message ordering in many cases. I need to think about this more, or maybe someone else has an idea. One new idea that came to my mind is:
* Always sort noticed messages under the newest info message (this PR sorts them under the newest noticed message, master sorts them purely by their sent timestamp)
* Always sort unnoticed messages under the newest noticed message (that's the same behavior as in this PR and on master)
* Always sort protection-changed info messages to the bottom (as in this PR)
However, after a talk with @link2xt we instead decided to add rule 3. (see above) because it seemed a little bit easier.
