The timeline can be vertically navigated by flicking, which is not very
appropriate for desktop applications. This change provides a scroll bar
and mouse wheel scrolling in addition to that. Also it makes the
vertical movement stop at the boundaries. The mouse wheel would exhibit
unintuitive behavior without it.

Change-Id: I5ef126525e452f46aa0a483a544345b8f618d829
Reviewed-by: Kai Koehne <kai.koehne@digia.com>