[racket] reparent gui object

I've added `reparent' as a method of `subwindow<%>'s. The original and
new parents have to be in the same eventspace.

My initial reaction to Jon's request was that it was too much trouble
to add `reparent', but sometimes I underestimate how much easier things
have become with the `racket/gui' rewrite. The platform-specific part
of the addition was trivial. Most of the work is just in checking that
a reparenting request is sensible (e.g., doesn't create a cycle).

At Fri, 21 Jan 2011 06:09:57 -0600, Robby Findler wrote:
> No, it isn't possible with the current GUI toolkit to do that.
> 
> Robby
> 
> On Thu, Jan 20, 2011 at 12:34 PM, Jon Rafkind <rafkind at cs.utah.edu> wrote:
> > Is there a way to change the parent of a gui object? (anything from the
> > framework collection)