If a column displaying text is empty, show the string "--" or the "Gedankenstrich" (a special longer dash). In order to distinguish this from an actual value of "--", we color it slightly gray (#999 or #ccc). We should probably discuss this further and try out different things.

The idea behind this change is to preserve the vertical layout of the column, in particular on sparsely populated lists and trees, to avoid that we have to introduce vertical lines to separate columns.