purple: Show self-messages for private messages only
Those are purple_conversation_write with PURPLE_MESSAGE_SEND flag set,
received through the write_conv UI op.
write_chat and write_im still receive and filter PURPLE_MESSAGE_SEND.
In the case of write_chat it *could* show some of those, but it seems
there's no decent way to tell echoes apart from remote self-messages.
So just keep those hidden for now.
