Restructure as MVC

[stuaxo]

This really needs some structure imposed on it, may as well go with MVC in the absence of other ideas.

See the end of this issue for references (there aren't too many when it comes to Gtk3 + MVC)

e.g.

preferences
    model.py
    controller.py
    view.py
example_menu_uimanager
    model.py
    controller.py
    view.py
example_menu_gmenu
    model.py
    controller.py
    view.py

Example detail for example_menu_uimanager:

XML for the UI would go in model
Event handlers for the UI go into controller
View builds the actual menu.

References:

[1] https://www.reddit.com/r/learnpython/comments/54wv97/learning_mvc_in_python_gtk3/

[2] https://github.com/wesleywerner/mvc-game-design
(referenced from 2)