Add Stop-Action to Tor notification

Mainly to stop failing or endless tor bootstrapping attempts.

There's one case when there's no other way to stop tor at all: If the vpn cert is expired but renewal fails (in the concrete case: due to ca cert transition). in that case tor+snowflake is started as a last resort, but only stopped on cert download failure, if no torified app is configured. A simple cancel button fixes that issue (and probably similar issues).