The integer user function GUI elements which can be None now are simply set to a wx.TextCtrl.
The wx.SpinCtrl used previously was incorrect, as a wx.SpinCtrl only handles integers, always
defaulting to the minimum of the control. Therefore the can_be_none user function arg arg is now
propagated into the wizard page Value, Sequence and Sequence_2D elements (though they are not
functional in the Sequence* classes yet).
