Introduce box2d debugging in perpetuity

Change-Id: If17f009dd39429c4bdaed45b028ac10d09176c53

qml/Gravity.qml

@@ -134,6 +134,15 @@ Item {
                 interval: 200
                 onTriggered: deathTimer.triggered()
             }
+
+            DebugDraw {
+                id: debugDraw
+                z: 1
+                world: physicsWorld
+                anchors.fill: parent
+                opacity: 0.75
+                visible: true
+            }
         }
     }