self_msg tag is not set on incoming messages

[arcnmx]

If a message was not sent by weechat, but was sent by your mxid, then the self_msg tag is not set for that line. This comes up in a few places:

• Messages that are sent from other clients / access tokens that aren't weechat (or are other instances of weechat on other machines)
• Messages that are populated from scrollback history from viewing a buffer, scrolling up, etc.
  • these messages also set tags like notify_private that make weechat treat them like new messages, should those be inhibited?