[v2] comctl32/taskdialog: Implement nDefaultButton and add tests

[Fabian Maurer]

I reworked my patch according to your suggestions.

>>> We already iterate over custom buttons and check every common button
>>> flag before this point, so it shouldn't be necessary to have another loop.
>>
>> I'm open to suggestion, but how would you do that?
>>
>> We could add three checks each time before taskdialog_init_button is 
called, 
>> or alternatively one check in taskdialog_init_button. Like giving it a 
pointer 
>> to default_button, and checking if the id equal the taskdialog default 
button.
>> But I find my current solution to be cleaner to be honest, what's your 
>> suggestion?
>Yes, something like that. taskdialog_init_button() has all the info to
> do that.

In the beginning I wanted to have default_button be -1 if no button was found 
yet, but as the tests show this is impossible since windows accepts a negative 
ID. Now we have to pass 2 pointers around..
Your thoughts on that? I find that the second loop makes things a lot cleaner.

Regards,
Fabian Maurer