Fix for issue #3089: Configuration Reference - Missing aggregation relations 

To be handle properly, this fix required an enhancement in the Introspection API,
to be able to get the aggregation relations FROM and TO a given managed object.

The following methods have been added to AbstractManagedObjectDefinition class :

getAllAggregationPropertyDefinitions()
getAllReverseAggregationPropertyDefinitions()
getAggregationPropertyDefinition(String name)
getAggregationPropertyDefinitions()
getReverseAggregationPropertyDefinitions()


Then, added the aggregation relations in the the Configuration Reference generated doc, 
in sections "Relations From this Component" and "Relations From this component" (today, only the composition relations are listed).

Note that the aggregation relations are not represented in the tree views on the left frame.
Any suggestion to do this is welcome!