Make restitution an option

Change-Id: If4c56d20a56e33a7e0af631fc4f361d4311eb662

qml/ArtDelegate.qml

@@ -13,7 +13,7 @@ ImageBoxBody {
 
     density: 1.0
     friction: 0
-    restitution: 0.1
+    restitution: settings.restitution
 
     fixedRotation: parent.fixedRotation
     bodyType: Body.Dynamic

qml/main.qml

@@ -18,6 +18,8 @@ Window {
         property double pace: 1.0
         property bool viewItemCount: false
         property bool globalWorld: false
+        // Very computationally heavy: 40% vs 20% for 0.1 vs 0
+        property double restitution: 0.1
     }
 
     Rectangle {