port 'Stanford PLY format' addon to Blender 2.8

Here are a couple of example files for testing: https://people.sc.fsu.edu/~jburkardt/data/ply/ply.html

I removed the texture support that was implemented here: rBA471370
(it is no official part of the file format; the exporter did not support it; reimplementing it is a bit of a hassle)

Reviewers: brecht

Differential Revision: https://developer.blender.org/D3754