GDK W32: Adapt to dest_surface removal, misc changes

GDK W32: Adapt to dest_surface removal, misc changes

* There's no GdkDragContext->dest_surface anymore.
  Add dest_window field to GdkWin32DragContext,
  and use that instead.
* Remove unused function prototypes
* Add more comments
* Rename variables and fields from 'window' to 'surface'
  where appropriate
* Fix header indentation a bit
* Try to ensure that uninitialized/unknown handle variables
  and fields are set to INVALID_HANDLE_VALUE instead of NULL,
  as there may be cases where NULL is a valid handle value.