Add a Way to Automatically Upgrade

[Shinmera]

There's a command to automatically pull new sources, but that doesn't update the binary parts. It should be possible to easily move on to the next release without losing all your projects and configurations or having to do it manually yourself.

[Shinmera]

I think the easiest way to do this would be to add a "VERSION" file that stores the latest major version. This can then be used to automatically construct the path to the download archive on Github without having to scrape any HTML files for the info.

As mentioned on the other ticket, doing this automatically would also require Windows to switch to a zip for the deployment as we cannot extract an exe automatically. I'm also not sure how things would work with upgrading the file while it's being executed. I think particularly on Windows that will be an issue.

[Shinmera]

Actually, GitHub features a releases/latest link that could be used, but has to be HTML-scraped to get the version out of. Should be a better idea than the VERSION file though since that file will otherwise get pulled in by a regular update that fetches the git versioned sources.

[Shinmera]

I'm not sure if this is worth the effort and potential for broken behaviour. Given what I've been able to tell so far there's multiple issues:

• Windows does not have a native tar.xz extractor and as such we need to have the initial ship in EXE format. The SFX I can produce don't allow specifying where to extract, so it cannot be fully automated. We could also ship a windows version as a ZIP or such-like, but the effort is probably not worth it.
• Getting the latest version requires scraping HTML which might break at any time.
• Replacing the sources while portacle is still running can and will cause problems. Tricks would have to be pulled to avoid that.

After all this I don't think it's a good idea to keep this for a 1.0 release. I might reopen this if I feel courageous, or if someone else has ideas on how to fix all of the above.