Update and allow assigning a new default user agent

The default user-agent for Ketarin is currently "Mozilla/4.0 (Windows; MSIE 6.0; Windows NT 6.0)", which represents a browser that's 15 years old and was replaced over a decade ago.

That is to say, it's very old.

Using a custom header (such as "Mozilla/4.0 (compatible; Ketarin; +https://ketarin.org/)") would be better, and having the option to globally replace the new default user-agent in the settings (probably on the Connection tab) would be ideal.

If you want to use a generic browser UA, then the safest would probably be "Mozilla/4.0 (compatible;)" as that is nondescript but still accounts for about 5-10% of all "real" browser traffic from newer browsers with pre-caching enabled.