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.
b089630b4c