fix magic menu in qtconsole with new user_exp

user_exp is now a dict based on mimetype, update qtconsole in
consequence

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Qtconsole frontend was making some assumption on the format on user expression to populate the "all magics" menu. This update it to work with https://github.com/ipython/ipython/pull/3319

Note to @minrk, this is a PR on your branch in your repo, I know that by mail it is not alway obvious when you get the notification by mail.