Core: revise property API (#490).

BREAKING CHANGES

The core service's property handling methods are no longer polymorph. Instead there are separate methods for topics and associations. The plugins and their APIs are adapted accordingly.
The formal parameter "key" is renamed to "propName". Its semantics have not changed.

Core Service:
    - getProperty(long objectId, String key)       -> getTopicProperty(long topicId, String propName)
                                                      getAssociationProperty(long assocId, String propName)
    - setProperty(long objectId, String key, ...)  -> setTopicProperty(long topicId, String propName, ...)
                                                      setAssociationProperty(long assocId, String propName, ...)
    - hasProperty(long objectId, String key)       -> hasTopicProperty(long topicId, String propName)
                                                      hasAssociationProperty(long assocId, String propName)

Access Control Service:
    - getCreator(objectId)                         -> getTopicCreator(long topicId)
                                                      getAssociationCreator(long assocId)
    - setCreator(long objectId, String username)   -> setTopicCreator(long topicId, String username)
                                                      setAssociationCreator(long assocId, String username)
    - getOwner(long objectId)                      -> getTopicOwner(long topicId)
                                                      getAssociationOwner(long assocId)
    - setOwner(long objectId, String username)     -> setTopicOwner(long topicId, String username)
                                                      setAssociationOwner(long assocId, String username)
    - getACL(long objectId)                        -> getTopicACL(long topicId)
                                                      getAssociationACL(long assocId)
    - setACL(long objectId, AccessControlList acl) -> setTopicACL(long topicId, AccessControlList acl)
                                                      setAssociationACL(long assocId, AccessControlList acl)

Time Service:
    - getTimeCreated(long objectId)                -> getTopicCreationTime(long topicId)
                                                      getAssociationCreationTime(long assocId)
    - getTimeModified(long objectId)               -> getTopicModificationTime(long topicId)
                                                      getAssociationModificationTime(long assocId)

=> If your plugin makes use of these methods it must be adapted.

In preparation to #490 (Property Index).
See #490.