New update check

Improved update check using even more cryptography ;)

Also supports end-of-life support for older releases using min-windows-version in version.txt.
This needs further considerations.
Problem: User on XP runs 1.8.10, we publish version.txt for 1.9 which requires Vista, now the users will see: "You are running the last version which is available for your OS" which is not true, as 1.8.12 is the latest version - but 1.8.12 is not linked/available using online update any more.