Reworking the nav bar, part 1.
This merges a lot of functionality into the navbar, shares painters, and
generally makes things more useful. It also changes the app to only have
one active zooming viewport at a time, which paves the way for simplifying
viewport logic all over.
Cleaning up viewport logic and adding frame navigation.
Adding back view visible range display on the nav bar.
Fixing time range drawing.
Fixing click-to-clear-filter.
Exporting begin/endTimeRange.
Fixing multi-tiered time range rendering.
Merge pull request #199 from google/navbar

Navbar rewrite.