[Importer] Allow column indexes in property primText

[MartinKl]

Problem

Sometimes it might be easier to specify the column index rather than the column name to identify primary texts. This not possible in the current version

Relevance

As an example, assuming I have a large number of files that I would like to import, but I did not assign a (uniform) title to my text column to which all the other annotation columns refer. The (gigantic) effort of properly naming that column in all files could be avoided by just pointing at the first column, for example.

Potential solution

To be most user-friendly I suggest to stick to column indexing by letters as used in common technology relying on the given file format. To indicate to the importer that a primText value is actually a column index, the value should be preceded by a #, e. g.

<property key="primtText">#A</property>

If you have thoughts or other suggestions, go ahead.