comctl32[6/8]: header: mark the callback items in a mask

[Mikołaj Zalewski]

Changelog: mark the callback items in the callbackMask HEADER_ITEM 
field. The pszText for callback texts is now NULL what simplifies some 
checks. Checks for lpItem->pszText==emptyString are also not needed as 
emptyString is not stored in lpItem but used as a parameter to 
Str_SetPtr (I haven't noticed it).
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