riched20: register ListBox & ComboBox classes

[Cihan Altinay]

Hi,

This patch partially implements the undocumented REExtendedRegisterClass()
function by registering the two missing classes. That way MS Office 2003 and
probably other programs that rely on these classes display dialogs completely
which were empty before.

Changelog:
    Cihan Altinay <cihan at uq.edu.au>
    Partially implement REExtendedRegisterClass() to register missing window
    classes so that applications relying on them don't get confused.