Make it easier to use backend sorting

[SpaceK33z]

Currently you have to do this to sort something on the backend:

const store = new Store();
store.params = {
    order_by: '-past_owner.last_name,-past_owner.first_name',
};
store.fetch()

I don't like this.

[SpaceK33z]

Idea:

const store = new Store();
store.backendComparator = ['-pastOwner.lastName', '-pastOwner.firstName'];

The advantage is that it looks a lot like the frontend comparator, store.comparator = 'asdf'. Still not sure about this though...