Work on OLE2-based generic DND

Intermediate commit of work in progress on integrating the old code
for OLE2-based generic drag and drop from Arhaeopteryx Software, from
a long time ago in the GTK+ 1.3 timeframe. Does still not work and is
as before not compiled in unless OLE2_DND is defined in
gdkdnd-win32.c. (Thus, for inter-process DND, still only WM_DROPFILES
style dropping of files on GTK+ apps works.)

Related slight refactoring of other code that shouldn't change how it
works. Add more global variables for run-time constants (once
initialized) representing well-known GdkAtoms and registered Windows
clipboard formats, as they with the generic DND code will be needed in
several source files. Some improved debugging output.

gdk/win32/gdkwindow-win32.c

@@ -1835,7 +1835,7 @@ _gdk_remove_modal_window (GdkWindow *window)
 }
 
 GdkWindow *
-_gdk_modal_current ()
+_gdk_modal_current (void)
 {
   if (modal_window_stack != NULL)
     {