Implement, and use internally, content_margins() from Widget.
Since AbstractScrollableWidget already has a method called content_size,
the convenience method, with the same name, in Widget has to be
explicitly called.
This commit adds a timer to AbstractScrollableWidget that can be used
when implementing automatic scrolling. By overriding
on_automatic_scrolling_timer_fired() we can calculate the scrolling
delta when dragging objects, and redraw as needed. A helper function,
automatic_scroll_delta_from_position() gives us a delta that
we can use to calculate speed and direction. By default
m_autoscroll_threshold is 20 pixels from the edge, and gives a linear
change in scroll delta.
