Fix of SPE-691 Slicer crash after extruder change

Added synchronization of GCodePreviewData between the front end / back end
(GCodePreview data is only used if PrintStep psGCodeExport is finished).
Added reset of GCodePreviewData on Print::apply() to conserve RAM.

src/slic3r/GUI/GUI_Preview.hpp

@@ -140,7 +140,6 @@ public:
 #endif // ENABLE_REMOVE_TABS_FROM_PLATER
 
     void set_number_extruders(unsigned int number_extruders);
-    void reset_gcode_preview_data();
     void set_canvas_as_dirty();
     void set_enabled(bool enabled);
     void set_bed_shape(const Pointfs& shape);