Design improvements to code workflow

Summary: Not really a specific issue, more a design to-do

Haoyi has a bunch of good observations in his article, Reimagining the Online Code Explorer. Most of them probably apply to Querki. Think about them carefully, and how they could be used to improve the overall code experience.

In particular, think about how to tighten up navigation and workflow. I've already thought about this a little (that we should have a preview pane for Large Text, for example), but I bet we could do vastly better. Just having an optional Models/Instances pane, to allow you to jump around, might make life much better. I think the key observation is that you are rarely editing a single Model at a time -- you are working in a larger workflow, and we should make that much easier.

Here's a possible loose design --

On the left-hand side of the Advanced Editor, we have a new Tree Pane. This is quite similar to the Show All Things view: a tree control showing all Models and (if you open them) Instances. This is on the narrow side, and on a small screen is folded shut by default.

On the right is the Advanced Editor Pane, as we think of it conventionally. But I can click on other Models, and quickly switch to their Editors.

Moreover, the right-hand side can split. If I click on an Instance, the lower half becomes the view of that Instance. There is a Refresh button over that, so that, as I edit the Model, I can refresh the Instance.

I can also create a new Instance of the Model on top, so that I can work with the Model and Instances relatively smoothly. When I finish editing an Instance, I see it rendered.

I can also create and edit a Page this way, editing it in the top half, and rendering it in the bottom.

None of the above is hard and fast design, but something like this seems likely to make it much, much quicker and easier to do complex editing flows.