don't scroll the message window when updating messages

[Martin Meyer]

Whenever Gwibber does a feed update the messages window resets its scroll position to the very top. This is annoying if I was trying to read messages while it automatically updates, or if I was trying to keep track of the last message in the stream I actually had time to read.

Can we make the scroll position only change if it's already at the very top of the message list? What I mean is, if I'm scrolled down below the first message I would like it to keep the current scroll position. I was probably scrolled there intentionally.