Towards more separation of GUI and data

[ebranlard]

Some of the issues:

• Testing the GUI is difficult, and wx fails on Travis.
• Too much interdependency of GUI/Data makes it difficult to potentially change GUI framework (qt for instance).

That's a continuous task, to:

• Isolate "generic" code that manipulates data, reducing the "GUI" code to layout, callbacks and calls to the "generic" code.
• Add more tests for the "generic" code.