CWM12 — supplierDependency reference is missing from ModelElement

The supplierDependency reference is missing from ModelElement, which existed in the CWM Adapted Specification.This change makes the model illogical and unbalanced. Dependency is always defined between two ModelElements (a client and a supplier). Its definition involves two associations and four association ends. Either the supplierDependency reference should be put back (which makes a more flexible model) or the client reference on Dependency should also be removed (which makes a more restrictive model, but at least it is logical and balanced). (This is a revised write-up for issue #3398.)