Currently we have

* `background` For background layers created by the background layer directive.
* `preview` For preview layers created by the catalog and import WMS directives.

The layer manager directive, for example, uses a layer filter function not to include preview and background layers in the layer manager UI. Other directives use such a filter function.

I think it's a good thing to separate concepts and use different properties for each type, even if this makes our filter functions a bit more complex. Having separate properties brings flexibility – we can write any type of filter functions. So for vector layers used for highlighting I think I'd just add a `highlight` property. In the future, and we'll have drawing tools, we'll add a `draw` property.

If we realize that many components need the same filter function then we could even add high-level, read-only, accessors in layers, expressed in terms of the lower-level properties. E.g.

```
  manageable: {
    get: function() {
      return !this.background && !this.preview &&
        !this.highlight && !this.draw;
    }
  }
```