get_option moved from Tab to ConfigOptionsGroup and extended. Added change_opt_value to changing option value in config

lib/Slic3r/GUI/OptionsGroup.pm

@@ -216,6 +216,7 @@ sub _build_field {
     return undef if !$field;
     
     #! setting up a function that will be triggered when the field changes
+    #! think of it as $field->on_change = ($on_change)
     $field->on_change($on_change);
     $field->on_kill_focus($on_kill_focus);
     $self->_fields->{$opt_id} = $field;