Add "About" menu item

[roata]

We need this feature so we have a place where we can give credit to all the open source components we rely on, as well as the people who have contributed to the project.

We shall encourage the authors of other forks to keep that window in their modified version of the software too.

Acceptance criteria:

1. New menu item called "About"
2. When that is clicked, show a window that displays some basic prose and a bullet list of references to other modules we are using.