Rewrite the renderer GUI using FABLE-elmish-react

[tomcl]

This is a suggestion for a future enhancement to the Visual2 code. I'm annoyed by the duplication in the HTML used for the skeleton, and also in much of the GUI F# code.

I suggest rewrite using fable-elmish-react

Advantages:

• Write the web part properly, making changes much easier
• Reduce code size by abstracting common patterns
• elmish-react is the referred F# SPA tech and looks to have a long life.
• elmish-react is at centre of new web development ideas and so tech learnt will have value beyond F#
• (for person doing it) a great way to get into fable-elmish-react

Additional reference:

fable-elmish-react-electron toolbox may help in integrating this with electron framework, though I suspect nothing there needs to change.

I will definitely do this over the Summer if no-one else bids - so if you are possibly interested indicate this on the issue. If someone else does it I'll provide code review etc.

[tomcl]

using react components I'd expect the following to be implemented functionally using maps and tables without duplicate code:

• List of settings (and code to persist them with electron-settings)
• List of registers
• set of buttons on toolbar

[tomcl]

The renderer mutables have got a bit more complex due to adding more caching to increase performance when single-stepping. The caching uses mutables to store intermediate results and avoid recomputation. It is effectively a crude form of memoisation and thus referentially transparent (and harmless) - but it does not look like that. Furthermore the additional mutables have global scope, if used wrongly they would not be harmless!

TODO: when the renderer code is refactored find common boilerplate (like a memoise function) to do this with embedded mutable state in a guaranteed transparent way. Then experiments can be made remembering arbitrary things to increase performance in a clean way, with the extra mutability hidden and harmless.