1) Changed the name of the variable "brim_offset" to "brim_separation"
for clarity.
2) Added legacy conversion after loading an old 3MF that does not define
then new "brim_separation" variable: The "brim_separation" is being
filled in with the "elefant_foot_compensation" value to produce
equal brim separation to the old PrusaSlicer that saved that 3MF file.
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