GGU-STABILITY: "Reference staff" menu item

Representation of a reference staff including horizontal auxiliary lines provides improved clarity with regard to terrain heights.

The position of the reference staff is entered as the distance from the left page margin in "x[m]". The height range in y-direction is given as "Top [m]" and "Bottom [m]". All values are in metres in the scale selected (see menu items in the "Page size + margins" menu).