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
Because the launch code uses ShellExec if the target is manifested the user will receive a prompt, so that will still work. ApprovedExes (which this bug did not mention) doesn't provide a means of specifying the working folder (as Sean mentioned) so it didn't make sense to allow this to be authored if it wouldn't be able to do anything. We would have to add an "Ex" method to the engine interface if we wanted to enable that.

Remember, this is WixStdBA code, not engine code available to all BAs.