comdlg32: itemdlg split button implementation

[Vincent Povirk]

These patches apply on top of my series that implements the open
dropdown menu in the common item dialog, based on BS_SPLITBUTTON. They
work in my testing on Windows, but they do not yet work on Wine
because BS_SPLITBUTTON is unimplemented.

This is mostly to show how to do this properly (and hopefully make the
case that the deficiency won't hinder development in the mean time),
but if I'm not around when BS_SPLITBUTTON is implemented, I consider
this a contribution to Wine under the LGPL just like anything I send
to wine-patches.

I'm interested in feedback in terms of whether this is as correct and
clean as it could be, but it's sort of academic until Wine has the
necessary architecture to support it.