Fix GUI freezing when wizard run from MicroDrop

Prior to this commit, GUI was freezing when the reference calibration assistant
was run from the MicroDrop GUI. This commit, seems to fix the problem.  Note
that any code that *might* be called from another thread *(e.g., a `idle_add`
callback)* must be wrapped as follows:

    gtk.gdk.threads_enter()
    ... code ...
    gtk.gdk.threads_leave()

See [here][1] and [here][2].

[1]: http://stackoverflow.com/questions/7850134/pygtk-thread-not-running-until-main-quits
[2]: http://faq.pygtk.org/index.py?req=edit&file=faq20.006.htp