Short-circuit in NavGroup.get_active

[HiPhish]

While doing #172 I also noticed potential for a small performance improvement.

• If the superclass check succeeds we do not need to check the items because the or in the return statement would short-circuit anyway regardless whether any of the items are active or not
• Since any returns True on the first success we do not need the entire list. Using a generator saves us from evaluating the remaining get_active calls. If any of the items has an expensive implementation of get_active we save ourselves from calling it.