Implemented extension of the G-code post-processor framework:

1) New environment variable SLIC3R_PP_HOST contains one of
   "File", "PrusaLink", "Repetier", "SL1Host", "OctoPrint", "FlashAir", "Duet", "AstroBox" ...
2) New environment variable SLIC3R_PP_OUTPUT_NAME contains the name
   of the G-code file including path (for SLIC3R_PP_HOST == "File")
   or a name of the file after upload to the host (PrusaLink, Octoprint ...)
3) The post-processing script may suggest a new output file name
   (likely based on SLIC3R_PP_OUTPUT_NAME) by saving it as a single line
   into a new "output name" temp file. The "output name" file name is
   created by suffixing the input G-code file name with ".output_name".

Please note that the G-code viewer visualizes G-code before post-processing.

Fixes Broken PostProcessing when script changes out-filename #6042

src/libslic3r/GCode/GCodeProcessor.hpp

@@ -337,9 +337,9 @@ namespace Slic3r {
                 std::string printer;
 
                 void reset() {
-                    print = "";
-                    filament = std::vector<std::string>();
-                    printer = "";
+                    print.clear();
+                    filament.clear();
+                    printer.clear();
                 }
             };
             std::string filename;