DefaultTimeline: Always show the vertical scrollbar

In "timelines", there *should* always be enough content to show it
anyways, and always showing it remove the weird size-jump when first
initializing a timeline.
UserFilterEntry: Don't forget the screen_name's @
Merge branch 'master' of https://github.com/baedert/corebird
timelines: Move auto scroll login into DefaultTimeline
DefaultTimeline: Don't always decrease the unread_count in mark_seen

If seen was true before, we shouldn't decrease the unread_count.
MentionsTimeline: Code style
timelines: Move more common code into DefaultTimeline
Corebird: Save open windows on quit

Related to #135