Better indicate occurrences

[nichtich]

Occurrences of a given concept should better be shown distinct from co-occurrences by adding a straight line or em-dash in the column of the second concept scheme so its more clear that only one concept scheme is references.

[nichtich]

To illustrate: In the last row it's unclear why there is nothing in the "to" column:

maybe add a mid-long line sich as "---" in the concept scheme column and the to column

[stefandesu]

This is specific for occurrences, right? Because mappings usually have to have a toScheme value (even if there is no concept in there). So I could check specifically if toScheme is empty and show the line in those cases.

[stefandesu]

Currently, if toScheme is empty (like in occurrences), it will be set to the target scheme if one is selected. That way, the "mapping" can still be saved directly. But maybe this isn't necessary, I can't really think of cases where this would be useful. I'd suggest to remove that feature and just show a line to indicate an empty target.

[nichtich]

In the example given above the value of toScheme seems empty, although it might be set to the current target scheme internally. At least the To row should be indicated as explicitly empty by a line.