Added machine evelope configuration parameters

(the MachineEnvelopeConfig class).
Added localization support for libslic3r through a callback
(the callback is not registered yet, so the localization does nothing).
Localized the Print::validate() error messages.

xs/xsp/Print.xsp

@@ -133,7 +133,7 @@ _constant()
     ~Print();
 
     Ref<StaticPrintConfig> config()
-        %code%{ RETVAL = &THIS->config; %};
+        %code%{ RETVAL = static_cast<GCodeConfig*>(&THIS->config); %};
     Ref<StaticPrintConfig> default_object_config()
         %code%{ RETVAL = &THIS->default_object_config; %};
     Ref<StaticPrintConfig> default_region_config()