We now support -gtk-win32-theme-part(class,part,state) in background
and border-image CSS properties. This renders the corresponding
theme part using DrawThemeBackground() and acts as a base for a
CSS based windows theme.

Note that we build the parsing code even on non-win32 so that
all themese will parse the same on all arches. We draw pink instead
of the actual theme parts on non-win32 though.