The GetProperty method is now just accessing contained data, meaning it
can be implemented in cmState.
Remove the cmMakefile context from the signature as a result and remove
the overload with the same signature.
Add a GetComputedProperty to cmTarget so that templates can be properly
instantiated. Otherwise the Commands would need to be able to reach the
specializations which are currently in cmTarget.cxx.
As a side-effect, the CMP0026 warning now gives a backtrace to the
target when issued from a generator expression.
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