[racket-dev] open untagged files in blank language level?

Okay, here's what I've done for the upcoming release. (It is a kind-of
patch on the existing setup, but perhaps that's appropriate, since the
right long-term solution is to  move everything to #lang.)

When a file is opened, and the language that would have been used is a
language that returns non-#f from get-reader-sexp (that means that
that language is one that adds metadata to the save file; currently
this includes the deinprogramm languages and the HtDP languages (and
possibly others)), then the language used is instead the
not-a-language language (that is the one you get when you have a fresh
preferences file).

In other words, I believe that this will have this effect:

  - after choosing a teaching language from the language dialog
  - and then opening a file that does not contain a prefix (no #lang prefix
    and no teaching languages prefix)
  - the language for that window will be set to the not-a-language language.

And hopefully no other effects.

Testing welcome!

Robby