Delete items rather than defining .display key

[KittyGiraudel]

It's a pain in the ass to keep items because then we can have types or groups that do have items but none of them is being displayed... Because of this we have to do very dirty stuff in templates.

Let's remove items completely.

[KittyGiraudel]

  eachItem(ctx.data, function (item, type, name) {
    if (!shouldBeDisplayed(item)) {
      delete ctx.data[type][name];
    } else {
      ctx.data.count++;
    }
  });

Also, theme needs to be updated.

[KittyGiraudel]

See #11.