Follow-up to 7c01ddf996

1) Starting with this commit, configuration block exported into G-code
   is delimited by "; prusaslicer_config = begin" and "; prusaslicer_config = end".
   These delimiters look like any other key / value configuration pairs
   on purpose to be compatible with older PrusaSlicer config parsing from G-code.
2) Config parser from G-code newly searches for "; generated by ..."
   comment over the complete G-code, thus it is compatible with various
   post processing scripts extending the G-code at the start.
3) Config parser from G-code parses PrusaSlicer version from
   the "; generated by PrusaSlicer ...." header and if the G-code was
   generated by PrusaSlicer 2.4.0-alpha0 and newer, it expects that
   the G-code already contains the "; prusaslicer_config = begin / end"
   tags and it relies on these tags to extract configuration.
4) A new simple and robust parser was written for reading project configuration
   from 3MF / AMF, while a heuristic parser to read config from G-code located
   at the end of the G-code file was used before.

src/libslic3r/GCode.cpp

@@ -1464,13 +1464,15 @@ void GCode::_do_export(Print& print, FILE* file, ThumbnailsGeneratorCallback thu
     	_write_format(file, "; total toolchanges = %i\n", print.m_print_statistics.total_toolchanges);
     _write_format(file, ";%s\n", GCodeProcessor::reserved_tag(GCodeProcessor::ETags::Estimated_Printing_Time_Placeholder).c_str());
 
-    // Append full config.
-    _write(file, "\n");
+    // Append full config, delimited by two 'phony' configuration keys prusaslicer_config = begin and prusaslicer_config = end.
+    // The delimiters are structured as configuration key / value pairs to be parsable by older versions of PrusaSlicer G-code viewer.
     {
+        _write(file, "\n; prusaslicer_config = begin\n");
         std::string full_config;
         append_full_config(print, full_config);
         if (!full_config.empty())
             _write(file, full_config);
+        _write(file, "; prusaslicer_config = end\n");
     }
     print.throw_if_canceled();
 }