Add notes to filter steps & data sets

[jelmervdl]

Now I'm actually using the thing, I'm noticing an urge to document why I'm adding filters and why I set the parameters as I set them. But there's no proper place for doing this.

It would be nice if I could add notes to each filter step. Just a text field that gets stored in the JSON and displayed in the interface (and can be hidden, presumably) would be sufficient.