cryptui: Add a partial implementation of CryptUIWizImport.

[Juan Lang]

> Yes, so those users may benefit from the stub as well. And I do print
> a FIXME. This is nothing new, we've been ignoring invalid certificates
> in wininet for years where we should stop and show a UI.

When I tested with native cryptui and imported a cert, it didn't pick
the root store.  So I'm just not convinced it's doing the correct
thing.  That the root store is at present read-only makes it even more
certain it doesn't do what the user expects at the moment.  Perhaps
that needs to be fixed as well, but that's a separate discussion.  We
need to be sure adding to the root store is the correct thing in the
first place.

I'm not against stubs, I just think this one needs some tests.  They
shouldn't be that hard to write:  test the root store before and after
importing a cert with no UI, and see if the cert is added to the root
store.  Try with a few different kinds of certs (self-signed, not
self-signed.)  If they're always added to the root store, I'll be more
persuaded.  Right now I'm not.
--Juan