New separate options for retraction length to be used when switching tools

lib/Slic3r/Extruder.pm

@@ -7,6 +7,7 @@ use constant OPTIONS => [qw(
     extruder_offset
     nozzle_diameter filament_diameter extrusion_multiplier temperature first_layer_temperature
     retract_length retract_lift retract_speed retract_restart_extra retract_before_travel
+    retract_length_toolchange retract_restart_extra_toolchange
 )];
 has $_ => (is => 'ro', required => 1) for @{&OPTIONS};