Only show Infobox for certain entities

Several users in the forum have been looking for the ability to only show the InfoBox for certain types of entities. For example, it would be useful to show the InfoBox for pins but not polylines.

One solution could involve this: #1592 (Picking Priority)

https://groups.google.com/forum/?hl=en#!topic/cesium-dev/H_-hJyY_KuA
https://groups.google.com/forum/?hl=en#!topic/cesium-dev/agtOrHYEhUo

Start by reviewing issue #1592 and the two linked forum discussions to understand the proposed selection and priority behavior. Define the entity-type visibility behavior described here, then verify that the InfoBox can be shown for pins while remaining hidden for polylines and other excluded entities.

Written by the indexing model from the issue text.