Recommendation is to avoid using the year nomenclature as this
information is already encoded in the git repo. Avoids needing to
repeatly update.
Also updates AUTHORS.txt from current repo with contributor names
Removed no-longer-required differentiation between importing
footprint vs board objects.
Renamed files to match the dialog.
Made Position At and Set Layer optional checkboxes.
Removed Group Items checkbox. (In the unlikely event that you don't
want a group, do an UnGroup after importing.)
Flattened out labelled-sizers in the dialog.
Removed importers blacklist, which hasn't been active for at least 4
years.
Fixed undo/redo bug that caused items to be no-longer-grouped after
a redo.
