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/src/libslic3r/I18N.hpp

@@ -0,0 +1,16 @@
+#ifndef slic3r_I18N_hpp_
+#define slic3r_I18N_hpp_
+
+#include <string>
+
+namespace Slic3r {
+
+typedef std::string (*translate_fn_type)(const char*);
+extern translate_fn_type translate_fn;
+inline void set_translate_callback(translate_fn_type fn) { translate_fn = fn; }
+inline std::string translate(const std::string &s) { return (translate_fn == nullptr) ? s : (*translate_fn)(s.c_str()); }
+inline std::string translate(const char *ptr) { return (translate_fn == nullptr) ? std::string(ptr) : (*translate_fn)(ptr); }
+
+} // namespace Slic3r
+
+#endif /* slic3r_I18N_hpp_ */