[WIP] Arrays of UI elements, type maintenance

[gustaphe]

I wanted to be able to set a table of sliders and get their values out as a collection, so I started rewriting src/Builtins.jl a bit.

But I needed some specific type functionality for that, so I started working on the types for the UI elements. Both adding an abstract supertype, making most of them parametric and adjusting the concreteness of parameters and arguments.

Turns out the functionality I originally wanted would behoove me to figure out how Pluto.jl deals with binding (and specifically javascript, so that's a project for the future). But the table printing turned out pretty sweet.

The Dict based types (Select, MultiSelect and Radio) are currently broken, both for some so far unknown reason, but also due to Dicts being unordered. I'll look into what to do about that at a later time.

(Don't merge, obviously, but I'm open for comments and suggestions)

[fonsp]

So far this only adds more scary complexity, making PlutoUI a difficult package to understand. I did not use an abstract supertype for this reason

You are right that we need a Pluto feature to support array things, once I have implemented that into Pluto I will update PlutoUI to match. See the discussion in #3 for more info

[gustaphe]

I get that. I think one could probably add a supertype with minimal scary complexity though, it doesn't with the current functionality need to be parametrized, so all you need is

abstract type AbstractUIElement end

and append a <:AbstractUIElement to each struct declaration. And the show and get function for arrays are then very simple (like the ones I implemented here). The "only" problem is the JSON parsing you mention in #3, I guess I underestimated the complexity behind the scenes. It could be possible to wrap the input table in a <form> and use the inputs js property. But it would require its own hook(?) and that's a bit beyond me at this point.

[fonsp]

Ah I only just now realized that you added the array feature. It's well found, but a feature like this should also work with custom inputs, like @bind x html"<input>", not just with the ones included in PlutoUI. I think I wrote something for this, let me check...

/~https://github.com/fonsp/disorganised-mess/blob/master/confirm%20combine.jl

Maybe we can put this in PlutoUI, with "confirm" and "combine" as two separate wrappers