[NbtkDraggable] use the absolute value when calculating drag delta
[NbtkDraggable] use the absolute value when calculating drag delta

Calculate the drag threshold using the absolute value of the delta,
so that the drag can move to the left and up as well (i.e. where the
delta would be negative).

Fixes bug 4430

jonnylamb/mx,jonnylamb/mx,pwithnall/mx,clutter-project/mx,pwithnall/mx,jonnylamb/mx,clutter-project/mx,clutter-project/mx,clutter-project/mx,jonnylamb/mx,pwithnall/mx,pwithnall/mx