Force `box-sizing` to combat reset templates which upset layouts.
Clean up example page for more space and responsive design tests.
Only need z-index on popup, adjust CSS to match.
Popup will always position itself inside viewport.
New display mode, `Widget.Dropdown`

A dropdown is tied to, and triggered by, a single field. The field is
given a temporary z-index so that it appears embedded within the
dropdown header. It replaces the calendar's title, if applicable. If the
widget is not entirely visible in viewport then document is scrolled.