Turn the Reference Font panel into a proper plugin

[justvanrossum]

This serves two purposes:

• it will help us refine the plugin API due to this being a real project
• it will allow is to make it more specific for the workflows common at Black Foundry

The current reference font panel can stay as long as needed, the plugin will effectively be a fork of it.

I've created a repo: https://github.com/BlackFoundryCom/fontra-reference-font