[racket] How to read custom editor-snip% from WXME file

From: Nick Main (david.nick.main at gmail.com)
Date: Wed Jul 24 18:03:01 EDT 2013

I created an extended editor-snip% class (also using the graph-snip-mixin)
and put an instance of this on a pasteboard% (altered
via graph-pasteboard-mixin) and then saved the pasteboard to a file.

Reading this file back into the pasteboard fails (no visible errors
either). Doing the same with a non-extended editor-snip does work, so the
only change is the extension class.

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