Calculate the position of the scrollbar handle correctly
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 <b84a892f57a298bb5dae653b4e188948a84d5549@digia.com>

mizukusak/qtquickcontrols,mizukusak/qtquickcontrols,mizukusak/qtquickcontrols