Add ability to set a layout manager scaling function via GUI

[SeanDS]

The layout manager scaling allows lengths in the view to be scaled so that large lengths (e.g. of the order 4 km) can be shown next to small lengths (of the order mm). Currently, this must be defined explicitly in the layout manager, but some way of setting this function visually or mathematically via the GUI would be nice.

This could possibly be built as a widget with a sigmoid function on a graph, with a "lower" limit and an "upper" limit in pixels, and a location of the half-way point on the x- and y-axis. The user can then drag the curve with the mouse and/or set these values in text boxes via the widget.