ENH: add timelapse gcode for multi_extruder

and add most used_extruder for print by object
jira:none

Change-Id: I021069e865992828b098d73c0d704ba7edbd55d8
(cherry picked from commit ae62f7b8e4b4258318c0d941963eddfb70e86c15)

src/libslic3r/GCode.hpp

@@ -332,6 +332,8 @@ private:
         const bool                       last_layer,
 		// Pairs of PrintObject index and its instance index.
 		const std::vector<const PrintInstance*> *ordering,
+        // idientiy timelapse pos
+        const int                        most_used_extruder,
         // If set to size_t(-1), then print all copies of all objects.
         // Otherwise print a single copy of a single object.
         const size_t                     single_object_idx = size_t(-1),