Check for updates and optionally auto-update

[RazinShaikh]

We should check for new releases in the background and inform the user. It would be even better if we could auto-update the app. This is an important feature in my opinion.

[jvdwetering]

Would the auto-updating work just by calling pip install zxlive --upgrade in the shell?

[RazinShaikh]

I don't expect most people to have installed zxlive via pip so ideally, it should check the GitHub releases.