Circular layout for dummy track

[etowahadams]

Currently, dummy tracks can only be used in a linear layout. Based on disucssion in #946 ideally we'd also support circular layouts as well. Here's a first attempt at supporting circular dummy tracks.

Challenges:

1. Tracks in a circular layout get compiled into a HiGlass spec differently than in a linear layout: Unlike when a view layout is linear, each of these Gosling tracks will not have a corresponding HiGlass view. Instead they will become HiGlass tracks within the same view.
2. Using a serial arrangement, tracks can be next to each other. We would want the same behavior with dummy tracks. See below for an example.

Implementation approach:
In general, the circular dummy track should be drawn with d3arc, which is also used in the brush track (brush-track.ts).