WIXFEAT:3249

Add ApprovedExeForElevation element, IBootstrapperEngine.LaunchApprovedExe, IBootstrapperApplication.OnLaunchApprovedExeCompleted, and enable WixStdBA to use them.

There are a couple of new pathutil functions that I had to keep in Burn, because I couldn't figure out how to get winterop and one other project to build. They couldn't find PathCanonicalizeW.