Switch automaticlly

[buhtz]

The user-agent should switch automaticly and (sometimes randomly) in different cases

• by definied time
• each n new clicks
• each n new tabs
• each restart of firefox
• in private-mode (see Add support for switching automatically to a different user agent in private browsing #7)
• on a per-domain basis (see Allow configuration of the user agent automatically on a per-domain basis #8)

When switching randomly there should be an extra list where I can select user-agents that can be used in the random-mode. Not all in the list make sense in some cases.