Design: Editor UI Customisation

This is a work in progress. Do not start implementing anything without discussing with @knolleary first

There are some requirements around allowing dynamic contributions to the UI.

Nodes can already contribute a custom panel to the sidebar. That is a stable API - albeit one that needs documenting.

We do not have an api available to contributing elements to the menus or buttons in the header. This design note looks at what is required and how it should be done.

Ideally, whatever apis are provided for content in the header, the existing UI elements should be updated to use them.