GDK W32: Special handling for DELETE requests

GDK W32: Special handling for DELETE requests

1) Ensure that any DELETE requests from the target are sent to GDK, even if
   both the source and the target are in the same process and it
   is therefore possible to use a shortcut and call the handler directly
   in GTK layer
2) Ensure that target GDK doesn't do anything when GTK asks it to send
   a DELETE request, just report back immediately (the code up the stack
   does not check for successfullness when request is DELETE, so not giving
   it any data is OK).

The source code already synthesizes a DELETE request, so that side is
also taken care of.

https://bugzilla.gnome.org/show_bug.cgi?id=786509