Condense and organize core loop

* Lots of DRYing
* polyline support for Entities
* Add icon and createInfoWindow support to filter functions
In `rset._pois['Entities']` the `poi` object has its Position in `poi['Pos']`. See here: https://github.com/overviewer/Minecraft-Overviewer/blob/792b049dd52ab575be87fbb97929589a0722109a/overviewer_core/aux_files/genPOI.py#L310