IsModifiedSavingPolicy will only try to save platform:/resource

Make sure the IsModifiedSavingPolicy will have the same behavior as
before by considering non platform:/resource URIS and .odesign files are 
not to be saved. This used to be tested in
org.eclipse.sirius.business.internal.session.danalysis.ResourceSaveDiagnose.isSaveable().
Activate IsModifiedSavingPolicy by default