Implement track style changes from #136

This is a rewrite of #205

Closes #205

Fixes #446
Small corrections to track style changes

- Remove comp-op src-over
- Remove line/background attachments
- Remove named bridgecase instance
- Correct comment syntax
- Remove opacity attribute for tunnels
- Decrease line-opacity of background
Merge pull request #557 from math1985/track_rewrite4

Track rewrite4