WIP: prusa-gcodeviewer command line wrapper to start the PrusaSlicer

in standalone G-code viewer mode.
Linux and OSX stuff will follow.

src/PrusaSlicer_app_msvc.cpp

@@ -221,6 +221,11 @@ int wmain(int argc, wchar_t **argv)
     std::vector<wchar_t*> argv_extended;
     argv_extended.emplace_back(argv[0]);
 
+#ifdef SLIC3R_WRAPPER_GCODEVIEWER
+    wchar_t gcodeviewer_param[] = L"--gcodeviewer";
+    argv_extended.emplace_back(gcodeviewer_param);
+#endif /* SLIC3R_WRAPPER_GCODEVIEWER */
+
 #ifdef SLIC3R_GUI
     // Here one may push some additional parameters based on the wrapper type.
     bool force_mesa = false;