WIXFEAT:3249 - Allow BA to run elevated async process through the engine.

Add ApprovedExeForElevation element, IBootstrapperEngine::LaunchApprovedExe, IBootstrapperApplication::OnLaunchApprovedExeBegin, IBootstrapperApplication::OnLaunchApprovedExeCompleted, and enable WixStdBA to use them.

Edit: WIP is at http://wixtoolset.org/development/wips/3249-allow-ba-to-run-elevated-aync-process-through-engine/
The .exe isn't "complete" (i.e., the process hasn't ended). I'm not sure how much value this string adds but if you want to keep it, it should be something like "Successfully launched approved exe..."