Enhance menu navigation

[bitpshr]

Currently, Parade uses a two-menu navigation approach in which the left navigation menu renders example widget names, and the right navigation menu renders example names and theme variations. As a result, we're losing out on ~400 pixels of screen real estate that's mostly blank on the right side of examples.

It may be nice to improve how Parade lays out its navigation menus:

• Expand widget names on-click to show example names as accordion children to the widget name.
• Move theme selection into the header.
• Remove the right navigation menu altogether.