[419926] Remove reference to old Viewpoint version

Change-Id: Ia491f911738f70049f1328115614e49cb41bf031
[422260] Remove Contribution duplicated classes.

Move classes from org.eclipse.sirius.viewpoint.description.contribution
to org.eclipse.sirius.description.contribution namespace.

Remove the generated copyright field.
Update the edit plugin id in genmodel and the editPluginClass to
minimize the regeneration differences.

Change-Id: I22186cd9992060f2a695f74236e6a51d2d4b0d4b
[cleanup] Use AbstractEList instead of BasicEList in
SiriusHelper.addUnique

do some cleanup in SiriusHelper.addUnique() to use AbstractEList instead
of BasicEList since we does not support anymore EMF 2.3 and now
BasicEList inherits of AbstractEList

Change-Id: I28600c96f4dbe4e01a9ed1bfbb3a9d1bf4c294cc
[clean] Remove an old comment

Change-Id: I484e4d2fa7e2c1d213d171e465e438783dc4afb5