refine member-added and member-removed plugin hooks to signal the sender (who added/removed a contact )

add ac_chat_modified hook event
add account.get_contact_by_addr (thanks @r10s)

src/chat.rs

@@ -2155,10 +2155,6 @@ pub fn remove_contact_from_chat(
                         msg.param.set_cmd(SystemMessage::MemberRemovedFromGroup);
                         msg.param.set(Param::Arg, contact.get_addr());
                         msg.id = send_msg(context, chat_id, &mut msg)?;
-                        context.call_cb(Event::MsgsChanged {
-                            chat_id,
-                            msg_id: msg.id,
-                        });
                     }
                 }
                 // we remove the member from the chat after constructing the