Catalog item composition vs Mixins

[nf-s]

I raise this in issue #4704

A heap of layers don't support ordering which can quite easily - for example any layers which create GeoJsonCatalogItems.
This opens up a broader issue of composition of different CatalogItems, and having to connect properties (for example show) for things to work properly.

Maybe we should create a GeoJsonMixin (and CsvMixin) so we can just shove them into a CatalogItem without having to worry about forgetting to define all of the properties.
I think this could speed up development time quite a bit and it would make simplify quite a few CatalogItems

I think I remember @steve9164 talking about how we should put as much functionality into Mixins and keep CatalogItems lean - is that right?

[KeyboardSounds]

Yep! This is exactly what Mixins are for. See: https://github.com/TerriaJS/terriajs/blob/next/lib/ModelMixins/Cesium3dTilesMixin.ts

[nf-s]

#5380

[nf-s]

Also, we should consolidate Mixins/Traits