non-blocking group QR joins (#2508)

* 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

examples/repl/main.rs

@@ -224,10 +224,11 @@ const CONTACT_COMMANDS: [&str; 9] = [
     "unblock",
     "listblocked",
 ];
-const MISC_COMMANDS: [&str; 10] = [
+const MISC_COMMANDS: [&str; 11] = [
     "getqr",
     "getbadqr",
     "checkqr",
+    "joinqr",
     "event",
     "fileinfo",
     "clear",