Exit to Menu

[thombruce]

I've just opened the game three times in a row and closed it again because I'd developed a habit of jumping straight in by hitting enter. I wanted to view the credits... 🤷‍♂️

Add a way to exit the game to the main menu, and add a way to exit from the menu to desktop without having to hit the 'X' or alt+f4.

The best way I can think of to enable this is navigable menus; you should be able to use cursor keys to navigate a menu and hit enter on an item to make your choice.

Exiting from game to menu will probably occupy the Esc key, meaning simple pause will either be removed or isolated to the P key. There will again be a navigable menu with "Do you want to quit?" as a prompt and "Yes" or "No" response options, or something loosely equivalent to that.

[thombruce]

See recent reworking of the Start Menu and Credits: #88

This PR introduced a new UI navigation system that would be really useful here. Consider having two buttons added to the pause screen:

1. Continue
2. Exit to Menu

With exactly the same approach taken in the above PR, this should be straightforward to implement.