Refactor OptionsGroup to decouple it from the config repository, add ConfigOptionsGroup

lib/Slic3r/Config.pm

@@ -96,13 +96,6 @@ our $Options = {
         cli     => 'g0!',
         type    => 'bool',
     },
-    'extruders_count' => {
-        label   => 'Extruders',
-        tooltip => 'Number of extruders of the printer.',
-        gui_only=> 1,
-        type    => 'i',
-        min     => 1,
-    },
     'gcode_comments' => {
         label   => 'Verbose G-code',
         tooltip => 'Enable this to get a commented G-code file, with each line explained by a descriptive text. If you print from SD card, the additional weight of the file could make your firmware slow down.',
@@ -912,7 +905,6 @@ sub validate {
                 qw(nozzle_diameter filament_diameter extrusion_multiplier temperature first_layer_temperature)
         );
     }
-    $Slic3r::extruders_count = $#$Slic3r::extruders + 1;
     
     # calculate flow
     $Slic3r::flow = $Slic3r::extruders->[0]->make_flow(width => $Slic3r::extrusion_width);