ETripleResource save should mark the resource content as saved

[Guillaume Hillairet]

The save method on ETripleResource translates its contents onto triples and send those to the RDF store.
Currently, when the save method is called, there is no mechanism that look at already saved objects, thus it always translate and save the all object graph.
Solutions:

• Turn saved objects into proxies, then when save is called again, only non proxy objects are translated and store.
• Test if object already exists in the RDF store.

[Guillaume Hillairet]

EObject that have been saved are now marked as proxies and will not be saved again unless a modification has been made to them.