runat-1128988: Add event.once
runat-1128988: Add event.once

I'm really surprised that this wasn't there already.

Signed-off-by: Joe Walker <7a17872fbb32c7d760c9a16ee3076daead11efcf@mozilla.com>

joewalker/gcli,mozilla/gcli,joewalker/gcli,mozilla/gcli,mozilla/gcli