Identify manual editions in automatically generated label files

Label files generated by a tagger can be manually edited and, if the resulting modified tagging is saved to file, the manual modifications should be identifiable.

This will be implemented with an orthogonal label: "edited".

Orthogonal labels are not supported yet. They require a dedicated attribute in the label file. Let us call this attribute "secondarylabels".

larvatagger.jl import with argument --default-label will automatically assign the orthogonal label to existing labels other than the specified default.

The GUI will use the orthogonal label on writing to file in the following circumstances:

• the input file is a label file, and
• the input file has "metadata" > "software" > "tagger" metadata, or the orthogonal label is already listed in "secondarylabels".

Only changes will be marked as editions.