Set silent flag for Sent carbons

[rufferson]

Silent flag (hint) in headers part of the message suppresses incoming message notifications hence needs to be set for any non-incoming message (such us outgoing carbon and mam).
https://telepathy.freedesktop.org/spec/Channel_Interface_Messages.html#Simple-Type:Message_Header_Key

silent (b)
    If present and true, the incoming message should not trigger a user notification.

https://telepathy.freedesktop.org/doc/telepathy-glib-0.24.x/TpMessage.html#tp-message-set-boolean

void                tp_message_set_boolean              (TpMessage *self,
                                                         guint part,
                                                         const gchar *key,
                                                         gboolean b);

Headers are in part 0.