Brim separation from object, follow up to 82373334bc

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.

src/slic3r/GUI/ConfigManipulation.cpp

@@ -281,7 +281,7 @@ void ConfigManipulation::toggle_print_fff_options(DynamicPrintConfig* config)
         toggle_field(el, have_skirt);
 
     bool have_brim = config->opt_enum<BrimType>("brim_type") != btNoBrim;
-    for (auto el : { "brim_width", "brim_offset" })
+    for (auto el : { "brim_width", "brim_separation" })
         toggle_field(el, have_brim);
     // perimeter_extruder uses the same logic as in Print::extruders()
     toggle_field("perimeter_extruder", have_perimeters || have_brim);