XML data should be opened by items and not by the layer.

[Fr3nchK1ss]

So far a layer reads the XML data of a graphicItem, then create the item with "new item_xxx", then add it to the graphical layer. The XML data should be read by the item itself and the code should be something like:
foreach (itemX)
addItemToLayer(new itemX(QDomData));

So each graphicsItem must implement a new constructor taking DomData as argument.