Refactored stairs rendering to support corners
Added some icons for people to use in their POIs.
Moved icons to subdirectory, adjusted docs

I also added one of the icons to the manualpois example, so people
have an exmaple of how they can use these in their configurations.
Added genPOI workaround for MystCraft

Possibly fixes issue #993.
Merge branch 'marker-icons' of https://github.com/CounterPillow/Minecraft-Libreviewer
Merge branch 'genpoi-mystcraft' of https://github.com/CounterPillow/Minecraft-Libreviewer
Merge branch 'corner-stairs' of https://github.com/superjer/Minecraft-Overviewer
Rebranding