feat: Don't send a notification when a group member left (#6575)

When there is a broken group (which might happen with multi-transport),
people want to leave it.

The problem is that every "Group left" message notifies all other
members and pops up the chat, so that other members also want to leave
the group.

This PR makes it so that "Group left" messages don't create a
notification, don't cause a number-in-a-cirle badge counter on the chat,
and don't sort up the chat in the chatlist.

If a group is deleted, then the group won't pop up when someone leaves
it; this worked fine already before this PR, and there also is a test
for it.

deltachat-rpc-client/tests/test_securejoin.py

@@ -653,12 +653,14 @@ def test_withdraw_securejoin_qr(acfactory):
     bob_chat = bob.secure_join(qr_code)
     bob.wait_for_securejoin_joiner_success()
 
+    alice.clear_all_events()
+
     snapshot = bob.get_message_by_id(bob.wait_for_incoming_msg_event().msg_id).get_snapshot()
     assert snapshot.text == "Member Me ({}) added by {}.".format(bob.get_config("addr"), alice.get_config("addr"))
     assert snapshot.chat.get_basic_snapshot().is_protected
     bob_chat.leave()
 
-    snapshot = alice.get_message_by_id(alice.wait_for_incoming_msg_event().msg_id).get_snapshot()
+    snapshot = alice.get_message_by_id(alice.wait_for_msgs_changed_event().msg_id).get_snapshot()
     assert snapshot.text == "Group left by {}.".format(bob.get_config("addr"))
 
     logging.info("Alice withdraws QR code.")