oceantv: event interface should not use fmt.Stringer but rather Name() string

We were deriving the event interface from fmt.Stringer so that events would have a String method and could provide their name, however, it's been realised that fmt.Stringer is probably not quite the right type of interface to be using, given that Stringer is more about returning the value of something as a string, not the name. This is more consistent with how we're (eventually) going to register events with a general purpose Registry, which will use a Name() interface to key registered "objects". If String() was used, then we would be keying based on the value of things, not their name.

Start by locating the oceantv event interface and the event implementations that currently satisfy fmt.Stringer. Change the interface contract to use Name() string, update affected implementations and callers, and verify that event names remain suitable for registry keys.

Written by the indexing model from the issue text.