Use multi-level argument names in demo_gen

[robertbastian]

Instead of flattening recursive parameters, like year, month, day if the argument is a Date, qualify them: date:month. Argument names are usually written in the context of a type, so when that is missing they are hard to interpret (e.g. name for the locale argument).

Single-argument names can be omitted, i.e. locale:name can just be locale.

[ambiguousname]

Overlaps with #611. I like this solution, but I also know @sffc had some opinions. Does this sound good?

[Manishearth]

I like this, though there's another option which is to store this as some form of metadata that the renderer can use, potentially to group things into regions, or use colons, or something else.

[robertbastian]

This is a low hanging fruit that doesn't require annotating all of ICU4X. Metadata can be built on top of it, but that's considerably more work. The current state is really not good.

[Manishearth]

Oh 100% we should do the easy thing first

[sffc]

I'm good with this, although I still think the better solution is to just name parameters less ambiguously.

[ambiguousname]

Ok, I'm on this when I have some free time

[Manishearth]

I like this, though there's another option which is to store this as some form of metadata that the renderer can use, potentially to group things into regions, or use colons, or something else.

Mockup of a good UI for this with <fieldset>