TaskDialog implementation

[Hans Leidekker]

On Fri, 2011-06-10 at 23:29 -0500, Vincent Povirk wrote:
> Well, I think you may be right that changing it to E_NOTIMPL at this
> point would break something and might be a bad idea. Sadly, Hans
> didn't say what the stub was originally for. Hopefully, we'll soon
> have a working implementation and it won't matter. ;) 

I added it for the Internet Explorer 9 installer but it has been
broken already when someone changed it to return IDYES instead of
IDOK in pnButton. I don't know which app that was for.