Categorical: don't sort the categoricals if Categorical(..., ordered=False)
In https://github.com/mwaskom/seaborn/issues/361 it was discussed that lexicographical sorting the categories is only appropiate if an order is specified/implied. If this is explicitly not done, e.g. with `Categorical(..., ordered=False)` then the order should be taken from the order of appearance, similar to the current `Series.unique()` implementation.
