* refactor: cleanup send_handshake_msg()
- rename to send_alice_handshake_msg() as used by Alice only
- remove dead code from Bob
(Bob's code is at BobState::send_handshake_message() since some time)
- take a contact_id and not a chat_id;
this makes things less confusing when
info-messages are put to the final group chat
* always directly return chat-id from dc_join_securejoin()
* take care not to create a group twice
* adapt documentation
* add info-msg on group invites; add inviter directly after creation
* document existing 'joinqr' command in repl tool
* do not create empty one-to-one chats for group-joins
* refactor: cleanup fingerprint_equals_sender()
- the function takes a contact_id directly now.
before it consumes the first contact of a one-to-one chat -
which may be easily confused with the group-chat in creation.
moreover, the conversion contact_id -> chat_id -> contact_id
is unneeded overhead.
* show info-messages in destination chat for alice
* fingerprint_equals_sender() returns Err on database failure
* tweak documentation
* clarify what an 'unfinished tasks' task is.
* add regression test for create_for_contact_with_blocked()
* rename Blocked::Manually to better fitting Blocked::Yes
* tweak test_secure_join() and make sure, Alice and Bob have only on chat after a group-join
* add basic multi-device-sync functions
* generate json
* add context.parse_sync_items()
* add context.execute_sync_items()
* piggyback sync-commands message, add body for human-readable part
* avoid double json renderings
* mimeparser parses incoming .json sync-files
* do not piggyback sync-files
* execute sync items
* return status of send_sync_msg()
* send sync messages as multipart/report
* add a per-item-timestamp and also allow adding other per-item-fields in the future
* if the self-chat does not exist, create it blocked/hidden
* create tokens closer to real qr-code needs
* respect bcc_self setting, add test for that
* sync qr code tokens after promoting groups
* send sync-messages only if an experimental switch is set
* trigger send_sync_msg() after sending messages and after creating/redraw/revive qr-code
* add DC_STR_* constants to deltachat.h
* adapt to refactored qr module as of #2729
* tweak test
* use SendSyncMsgs config name instead of SendExperimentalSyncMsgs - we can remove or rename the config nevertheless, but have the option to keep it without renaming
* tweak docs
* remove currently unused effective timestamp calculation
* clarify when send_sync_msg() is called
* make sure, sync-messages are encrypted and are sent by SELF
* tweak docs, fix typos
