Add Dropotron (Dropdown navigation menu) again

[suchja]

One of the features in the original theme by HTML5 UP is the Dropdown menu in navigation bar. So far I removed (see a02538b ) this for Statiq, because I could not think of any way to properly realize this here. The problems I see are:

1. what type of content would be displayed in such a navigation panel?
2. how do I tell Statiq which parts shall go into a nested navigation?
3. does Statiq support this at all?

Maybe there are even better ways of doing this, but I have no idea.