Abstract base class for property handlers to handle the (probably common) checks:
- the availability of the target id,
- multiple definition of a property for the same element
Stores the meta values in a map inside the metaDataMap. The keys of the inner map are the property names and the values are the stored meta values. If the property allows multiple definitions for the same element the values are store in a list, otherwise the content returned from the content processor is stored. Assumes that the only acceptable type is the default type.