add infinite line to default scene to display world center

Author: hugtalbot

examples/Demos/fallingSOFA.scn

@@ -30,7 +30,10 @@
     <VisualStyle displayFlags="showVisual"/>
 
     <!-- Define Mouse left click as a Bilateral Lagrangian constraint -->
-    <ConstraintAttachButtonSetting />
+    <ConstraintAttachButtonSetting/>
+
+    <!-- Drawing the world coordinate axis -->
+    <LineAxis  infinite="true" vanishing="true" thickness="1"/>
 
     <!-- Header of the simulation -->
     <!-- This animation loop will add steps in the time intyegration to enable Lagrangian-based constraint solving