Clarify API for PropertyValue, ObjectValue, ListValue

Clarify the API, e.g.:

• getPropertyValue always returns a PropertyValue
• getValue always returns a (resolved) String.
• remove Collection<PropertyValue> getValues(Predicate<PropertyValue> predicate). It can be implemented easily by clients.
• remove PropertyValue toPropertyValue(). Instances already are of that type.
• add ObjectValue setValues(Map<String, String> config) to ObjectValue..
• add-Methods already return the objects added, whereas set-Methods always returned the instance the call is operated on (builder style).
• add public static ObjectValue from(Collection<PropertyValue> values) on ObjectValue.