Catch dangling property ties.

Catch dangling property ties.
It's bad when tied properties are not untied before removing the tied property list.
We could try to "untie" the properties in the destructor - but that usually
caused weird mem access errors, since the tied properties will access
a no longer existing object (the very object whose destruction called the
TiedPropertyList destructor...). => so just add a warning and a nagging trap :)