Core: indexing property values (#490).

BREAKING CHANGES

The core service's set-property methods got a 4th boolean parameter: `addToIndex`. If set to `true` the property value is indexded and thus available for being queried later on. (The actual query methods do not yet exist.)

Core Service:
    - setTopicProperty(long topicId, String propName, Object value, boolean addToIndex)
    - setAssociationProperty(long assocId, String propName, Object value, boolean addToIndex)

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

COMPATABILITY NOTE

Regarding the standard plugins these properties are now indexed:

Access Control: "creator", "owner"
Time: "created", "modified"

=> The "creator" and "owner" values of topics/associations existing in a DM 4.1 installation are not indexed retrospectively.
=> The "created" and "modified" timestamps are not added retrospectively to existing topics/associations of a DM 4.1 installation.

See #490.