* allow deleting referenced contacts in UI
we are quite often getting requests of users
who want to get rid of some contact in the "new chat" list.
there is already a "delete" option,
but it does not work for referenced contacts -
however, it is not obvious for users that a contact is in use,
esp. of some mailing list or larger chat, old contacts, whatever.
this pr revives an old idea [^1] of "soft deleting" referenced contacts -
this way, the user can remove the annoying entry
without the need to understand complicated things
and finally saying that deletion is impossible :)
once the contact is reused, it will reappear,
however, this is already explained in the confirmation dialog of the UIs.
technically, this pr was simpler as expected as we already have
a Origin::Hidden, that is just reused here.
[^1]: https://github.com/deltachat/deltachat-core/pull/542
* update rust doccomment
* update changelog
* avoid races on contact deletion
chats may be created between checking for "no chats" and contact deletion.
this is prevented by putting the statement into an EXCLUSIVE transaction.
* fix failing python test
