Blocking visibleChanged signals from children during deconstruction

[uwerat]

When deconstructing a QQuickItem all children are becoming reparented ( -> no parent item ), what might lead to visibleChanged signals. When the parent is monitoring the visibility of the children using this signal the connected slot will be called in an almost deconstructed state usually ending up as crashes.

To avoid this situation all layout classes have to disconnect manually in the destructor.

A better solution would be if we could handle this problem in a more general way in ~QskQuickItem.