GDK W32: Force correct mouse cursor for custom resize/move

GDK W32: Force correct mouse cursor for custom resize/move

* Explicitly grab the device, setting appropriate cursor on it.
* Fix gdk_device_virtual_set_window_cursor() to just set the
  cursor, without trying to check that mouse is over the given
  window. Also prevent it from immediately resetting cursor.
* Alse take into account things that happen in other parts of
  GDK - don't look for replacement cursor, GDK already did that,
  and don't create a default arrow cursor instead of NULL,
  GDK-W32 already did that up the stack as well.
  Warn about inappropriate cursor == NULL argument instead.

https://bugzilla.gnome.org/show_bug.cgi?id=762711