get columns() {
    return this.modelType.columns.map(({ desc, name }) => ({
      valuePath: `columnValues.${name}`,
      name: desc,
    }));
  }
The lists's schema containing info about the list's columns.
This is usually a static object except in this case each model
type has different columns so we need to build it dynamically.

The format is:
```js
  [{
    valuePath: 'title',
    name: 'Title'
  }]

@property schema
@type {Object}