Allow changing the active timeline with the keyboard

The simplest way of doing it is by binding the Up and Down keys with the changing of focus to the timeline above or below, respectively, the current active timeline.

If this implementation is chosen, then the view on the QGraphicsScene object must be changed, in order to make the selected timeline always visible.

Also, allow cycling over the timelines, when the uppermost or lowermost timelines are reached and Up or Down is hit, respectively.