cmTarget: Move sanity checks and computed property access to callers
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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