Messages: allow avoid to listen to own messages

[panManfredini]

To make easy communication between custom elements one could immagine to register all of them to a message channel, so all of them will get the payload when posting a message to that channel.

modifying like this would work better:

on_update_message(event){
if(this === event.target) ---> do nothing
}

set a flag in the event detail to turn on/off this behavior.