Redesign of the model list GUI element.

The wx.ListCtrl element has been replaced by a wx.FlexGridSizer combined with wx.CheckBox and
wx.StaticText.  The result is a much nicer formatting of the element.  The checkboxes in the old
element displayed slight rendering problems on all operating systems and did not look neat.  The new
design is also more flexible in that models of None are now treated as separators in the window.
The model list GUI element can now display an optional model description column.
Added model descriptions and adjusted the size of the model-free model list GUI elements.
Refinements for the model list GUI window.

The font for all text elements is now set.  And the elements of the wx.FlexGridSizer are now
vertically centred so that the text of the checkboxes and text elements line up perfectly.