Superheroes and the ball are elements with the class "draggable". Make them really draggable.
Important: limit dragging by the window. If a draggable reaches window top or bottom, then the page should scroll to let us drag it further.
If your screen is big enough to fit the whole document -- make the window smaller to get vertical scrolling, so that you could test it.
In this task it's enough to handle vertical scrolling. There's no horizontal scrolling usually, and it's handled the similar way if needed.
And one more thing: heroes may never leave the page. If they reach the edge of the document, no dragging outside of it.
