Add LaunchWorkingDir to wixstdba to facilitate processes that require a different working folder

[WIXBUG:3750](http://wixtoolset.org/issues/3750/)
Separate pull request created for wix4
My point was that the doc describes what this attribute doesn't do, not what it does do. Something like

>WixStdBA will use this working folder when launching the application specified by the LaunchTarget attribute. (Specifying a working folder for LaunchTargetElevatedId is not supported.)

If that's what happens.