Calculate the position of the scrollbar handle correctly

When the width of the flickable in the ScrollArea was changing dynamically
so it ended up with an originX that was not 0 then it would not calculate
the leftMargin of the scrollbar correctly. The calculation here is done
based on how QStyle calculates it for consistency.

Change-Id: Ieb247f77ab10eba4015eed897d3babb94c752743
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>