Fixed #735 & PresetHints.cpp is marked to localization

* Macro _LC is created to put translated string into std::string correctly.
* Macro _LU8 is changed to function L_str.
* Created function from_u8

xs/src/slic3r/GUI/GUI.cpp

@@ -555,4 +555,14 @@ AppConfig* get_app_config()
 	return g_AppConfig;
 }
 
+wxString L_str(std::string str)
+{
+	return wxGetTranslation(wxString(str.c_str(), wxConvUTF8));
+}
+
+wxString from_u8(std::string str)
+{
+	return wxString::FromUTF8(str.c_str());
+}
+
 } }