qml: avoid direct reference of root control background and content items

Aliasing properties expects the referenced item to be there. However, contentItem and background items can always be overridden when the control is reused. In that case, this seems to be an undefined behavior. I have experienced crashes by aliasing a non-existing object's property.

I also remember reading at some place in the Qt docs that setting the id in these cases prevents some kind of optimization. (I no longer have the link to provide, unfortunately).