[v3 04/12] comctl32: Extend TaskDialog, add simple callback and tests

[Fabian Maurer]

> There's no reason to introduce global variable for that, just to remove
> it two patches after.
It's not split the functionality into two patches. I could merge them if you 
want, but I thought it would be easier to understand if I separated those two 
logical changes. First getting it to work with a global variable, then change 
it into something thread-safe.

> For minimal working replacement of MessageBox callback support is not
> needed, right?
No it's not needed. But, we certainly want the tests, don't we? Without 
callback, there'll be no tests.

Regards,
Fabian Maurer