When the sortables offsetParent has `position = fixed`, by dragging a
sortable, the cloned element being dragged has the wrong position if
the window or offsetParent is scrolled.
This PR adjusts that by using window and offsetParent scroll to adjust the position value.
Added also a new function to calculate better the position to work as
expected in cases where container has scroll, and position is fixed,
relative or absolute
The buggy behaviour can be seen in this jsFiddle: http://jsfiddle.net/B5yn5/
Fiddle example with the correction: http://jsfiddle.net/Hsf6W/1/
( closes #1243 | fixes #1242 | fixes #1113 | fixes #1191 )