zValue tweaks

- Sort by Z before duplicating a selection (so that relative Z's are
kept)
- in UBZLayerController, iterate only through UBGraphicsScene's fast
access items, not QGraphicsScene::items

src/domain/UBGraphicsScene.cpp

@@ -302,7 +302,7 @@ void UBZLayerController::shiftStoredZValue(QGraphicsItem *item, qreal zValue)
  */
 bool UBZLayerController::zLevelAvailable(qreal z)
 {
-    foreach(QGraphicsItem* it, mScene->items()) {
+    foreach(QGraphicsItem* it, dynamic_cast<UBGraphicsScene*>(mScene)->getFastAccessItems()) {
         if (it->zValue() == z)
             return false;
     }

src/domain/UBSelectionFrame.cpp

@@ -258,9 +258,20 @@ void UBSelectionFrame::remove()
     updateRect();
 }
 
+static bool sortByZ(UBGraphicsItemDelegate* A, UBGraphicsItemDelegate* B)
+{
+    return (A->delegated()->data(UBGraphicsItemData::ItemOwnZValue).toReal()
+            < B->delegated()->data(UBGraphicsItemData::ItemOwnZValue).toReal() );
+}
+
 void UBSelectionFrame::duplicate()
 {
     UBApplication::undoStack->beginMacro(UBSettings::undoCommandTransactionName);
+
+    // The mEnclosedtems list items are in order of selection. To avoid losing their
+    // relative zValues when duplicating, we re-order the list.
+    std::sort(mEnclosedtems.begin(), mEnclosedtems.end(), sortByZ);
+
     foreach (UBGraphicsItemDelegate *d, mEnclosedtems) {
         d->duplicate();
     }