If the property value must be one of a set of known tagged values,
then this method should return an array of the tag values. This can
be used to represent (for example) enum values. If a PropertyEditor
supports tags, then it should support the use of setAsText with
a tag value as a way of setting the value.
Returns:
The tag values for this property. May be null if this
property cannot be represented as a tagged value.