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

src/constants.rs

@@ -24,7 +24,7 @@ pub static DC_VERSION_STR: Lazy<String> = Lazy::new(|| env!("CARGO_PKG_VERSION")
 #[repr(i8)]
 pub enum Blocked {
     Not = 0,
-    Manually = 1,
+    Yes = 1,
     Request = 2,
 }
 
@@ -385,7 +385,7 @@ mod tests {
         // values may be written to disk and must not change
         assert_eq!(Blocked::Not, Blocked::default());
         assert_eq!(Blocked::Not, Blocked::from_i32(0).unwrap());
-        assert_eq!(Blocked::Manually, Blocked::from_i32(1).unwrap());
+        assert_eq!(Blocked::Yes, Blocked::from_i32(1).unwrap());
         assert_eq!(Blocked::Request, Blocked::from_i32(2).unwrap());
     }