do not send DC_EVENT_MSGS_CHANGED or DC_EVENT_INCOMING_MSG for hidden messages

these events take the message-id as parameter and might be used
to update an existing list (although to recommended)

if the event is issued for hidden messages,
this might led to "empty" messages flashing up -
the ui tries to get the message from the event,
after a moment, on the next update, the message disappears again
as hidden messages are of course not returned eg. by dc_get_chat_msgs().

the effect was probably always visible for secure-join-messages on ios,
however, become much more visible recently when read-receipts are added
as hidden messages as well (to make them auto-deletable).

src/dc_receive_imf.rs

@@ -674,7 +674,7 @@ fn add_parts(
     );
 
     // check event to send
-    if chat_id.is_trash() {
+    if chat_id.is_trash() || *hidden {
         *create_event_to_send = None;
     } else if incoming && state == MessageState::InFresh {
         if from_id_blocked {