add a create-flag to the node() method of a PropertyObject,
defaulting to false to maintain existing behaviour.
This could be used to add a listener to a non-existing property
without having to write a dummy-value beforehand.
Usage:
myPropertyObject->node() returns the corresponding node or NULL if
does not exist or has not been accessed before.
myPropertyObject->node(true) returns the corresponding node, never NULL.
If the property does not exist, it will gets created.