Revert "QmlDesigner: Remove NodeInstancesSignalSpy"

This is fixing some anchor movement bugs.

This reverts commit ae409f8bdf070cc7875da1896abcdaa077f5e932.

Change-Id: Ibe9b8b2601e2bcd1c249b5feda06fb3a29482f4b
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>