Composite: setting properties of the main implem

[snybril]

It is possible to override the relations of the main implem,
but not its initial properties ?

It seems especially useful when combining contextual constraints and navigation.

[snybril]

May be not released with milestone 0.0.6 (change with the schema)

[snybril]

Wiil be done in 0.0.8, the child element is there but not used at runtime in 0.0.7