Enhancements/Bug fixes for prokaryotic summary visualization project

[ThHarbig]

This issue tracks all issues related to bugs that need fixing or useful enhancements for the prokaryotic summary visualization project.

Bugs:

• Brush display in stacked tracks in circular layout #885
• Displacement type pile does not work #883

Enhancements:

• Create API to support summary visualizations in gosling #895
• Improve ways of gosling to communicate with the outside world #894
• Metadata concept in gosling.js/gosling-meta.js #925
• Placeholder views #924
• Make higlass view specs invisible in API #909
• Unify brush track and rangeSelect mouse event #910

Wishlist (enhancements that would be nice to have for the examples that I implemented):

• Overlaying single tracks across multiple tracks #884
• Reducing number of data redefinitions #508
• Click to change brush position #889
• Clockwise orientation of circular layout #280
• Advanced triangle shape #905
• clockwise orientation of circular layout #280