I thought that `len(categories)` is expensive to call only for checking amount of displayed categories. In that property we create several  instances of EverythingMeCategory class, while with `categories_count` we simply check length of list returned by marionette. In some tests we don't need any `category` as a AppObject.

Perhaps, I can rewrite it to just return True if there is at least one category of False if there are no categories.