Class based plugins API

**Describe the enhancement**

Currently plugins work by developers providing module level functions with particular names, this is simple but can make it difficult when we want to alter the plugin API

My personal preference is for plugins to implement a base class that we provide. That should also make it easier for us to do things like the "per plugin logging" and others. It can also give us more control over how we present functions that have to run on the main loop or can run in a thread.

I've raised this as a placeholder for discussion.

No files, tests, or entry points are named. Start by reviewing the existing module-level plugin API and the discussion about a base class, per-plugin logging, and main-loop versus threaded functions. Done would require an agreed class-based plugin API and an implementation plan.

Written by the indexing model from the issue text.