account: Reduce unwanted scrolling

"size-allocate" signal can be triggered even when the revealer's state doesn't change, so we need to check if the revealer is actually revealing.

The 'obvious' solutions for this don't work. If we connect to "child-revealed", the scroll only happens after expanding the revealer. If we connect to "reveal-child", we'll only scroll to the bottom at the beginning of the revealer's expansion, so the list will grow longer without scrolling.

The handling is still imperfect though, since we always scroll to bottom regardless where the revealer is.

Improves #450 (closed).