use part/join format for (dis)connects

[fauno]

when a person connects and disconnects, i receive a notification like this:

13:33:37    --   person just went offline.

while on other channels it'll be like this:

13:06:15             <--   person (~person@hostname) has left (Ping timeout: 264 seconds)

the issue is that with smart filters, you don't see the parts/joins unless the person has been talking during the last 5 minutes, but since tox notifications are in another format, they're always printed, making the unread lines counter on the buffer bar increase.

i think you can put anything on the right side, you just have to use the arrows.

i'd contribute patches but my C is awful :(