Provide a secure method of upgrading Fury

[propensive]

There should be a fury upgrade option in the menu which checks for new releases on https://fury.build/. If a newer version is available, it should be placed in a special location. The launcher script should check that location before starting. If there is a file present, it should replace itself with the new version, and exec that (passing in all the same parameters).

Downloading should verify the downloaded file's checksum using a public key on Fury's classpath. Built artifacts should be distributed with corresponding .sig files.

[propensive]

This check should only be done when launching Fury; not for each invocation.