Paned Tree UI

[georgefst]

(This is really just a placeholder for now, so that I can link to it from other discussions. It's not intended to be comprehensive.)

The rigidity of positioning of definitions on the canvas (a row for types, a row for terms, each sorted alphabetically) is at this point perhaps our biggest single UX issue. While there are small improvements we could make to the existing setup, such as allowing re-ordering via an action, it's clear that we ultimately need a new approach.

We had until recently always assumed that we'd eventually want to just make better use of 2D space on the single canvas, either by automated packing or by allowing defintions to be dragged around freely. But we have recently discussed, particularly at our 2023-08-16 Developer meet-up, the idea of moving to a multi-pane UI with each pane displaying a single definition. We believe this would be easier to use (and implement!) than interacting with all definitions via a single canvas.

There are still many details to flesh out.