NEW: add gcode conflict checker

1. add gcode conflict checker
2. add a new command line option: --no_check. When it's provided, gcode
   conflict check is skipped.

Change-Id: I6feafca8c5fa6c3c5eae1f0e541ce59d2f03dedf
(cherry picked from commit 5a39afb64e826a960d9673dcada89d02d62b3911)

src/libslic3r/Print.hpp

@@ -79,7 +79,8 @@ enum PrintStep {
     // should be refreshed.
     psSlicingFinished = psSkirtBrim,
     psGCodeExport,
-    psCount,
+    psConflictCheck,
+    psCount
 };
 
 enum PrintObjectStep {