list of subnode should be dynamically computed

[NicolasAnquetil]

either by offering a callback on the expand action (that would allow a client to recompute the list)
or by allowing a block for the list of subnodes

[bergel]

I believe we could do this by creating subclasses of HNode specialized in what it should be open (e.g., package)

[NicolasAnquetil]

Not sure, because even if we know it is a node for a Package, there may be different kind of sub-nodes that the user would want to have.

I really believe the sub node should be:

• [Pharo way] computed from a block (taking the hnode rawModel as argument), or
• [Java way] recomputed by the user every time it is needed, and this would require a callback from the visualization to some method provided by the user

The first one seems a lot better

[bergel]

Related to #49