Add event notifying that realtime channel advertisement is received

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When send_webxdc_realtime_advertisement is called, an advertisement mail is sent over SMTP. For bots it would be useful to know when such mail is received so it knows some user has opened a webxdc that uses realtime channels and bot might want to connect to the "swarm" and send its own advertisement. Otherwise bot managing a lot of webxdcs does not know when it should advertise its own address after restart.