PreView v2020: 2D Font substitution table

2D drawings may contain a set of fonts, which are typically not included with the drawing file, but which need to be installed on the local system in order to render the drawing correctly. So far we allowed to substitute all fonts by a configurable single one. 3DViewStation now will prompt, if one or several fonts could not be found and allows to maintain a font substitution table, assigning different fonts to each one, which could not be found.