Map/3D view parameter export: not always the visible parameter is exported.

Technically, it is only required to select a layer (yellow rim) and then press the right mouse button and select “Save property as grid” in order to export displayed values to a file. In 3D view, however, only the content of the shallowest/youngest layer is saved if more than one layer is selected at the same time. In this multi-selection case a re-selection of only one layer has to be done to pick the desired horizon and export its properties.