Position of the next widget to add. If some widgets are added one after another,
it is similar like a flow-layout.
But the position can be set.
The values inside the position are positive in any case, so that the calculation of size is simple.
protected boolean posUsed
False if the position is given newly. True if it is used. Then the next add-widget invocation
calculates the next position in direction see GralPos.setNextPosition().