Support highlight style rules

[josephbirkner]

Currently, highlighting is done via Cesium's native feature color re-assignment. However, this will not be possible for arrows/dashed lines. Furthermore, we want to support rules which display information only when the feature is selected. For this use-case, we introduce the mode: hover|highlight field, and the selectable: true|false field. Note: A visualization generated with the highlight condition will never be selectable itself.