I think the Model-View-Presenter pattern is better for this than the
Model-View-Controller pattern.

The name "controller" is used in the MVC pattern context here, so we
should rather go with another name.

I decided to go with the MVC (Model-View-Controller) pattern. This is
the first step by making MainWindowContainer a MainWindowView.

Although it isn't necessary for Python 3, some tools analyzing code
(e.g. pyreverse from pylint) need those files.

As per http://adrienplazas.com/blog/2020/10/15/specify-form-factors-in-your-librem-5-apps.html

Minimum percentage for briar-gtk: 60%

Fixes #11

Fixes #29

Closes #14