ENH: add load_model_objects_and_custom_gcodes

Add cmd line option to pass layer change gcode.
This is useful for lithophane auto change colors.
Cmd: --custom-gcode custom_gcode_toolchange.json

Examples of the input json file can be found in the jira.

Jira: STUDIO-4070

Change-Id: I5beb5ff7d6d81028e95013e79f955e498cd3ba30
(cherry picked from commit bed6152b95c965ea87958d94ec069d8c883a8037)

src/libslic3r/Config.cpp

@@ -756,6 +756,7 @@ int ConfigBase::load_from_json(const std::string &file, ConfigSubstitutionContex
     try {
         boost::nowide::ifstream ifs(file);
         ifs >> j;
+        ifs.close();
 
         const ConfigDef* config_def = this->def();
         if (config_def == nullptr) {