Add support for PLY proxy objects

Planned workflow:

• Select object that has a long export time and/or makes OpenGL viewport sluggish (e.g. trees)
• Press a "make proxy" button (or a menu entry in the export menu?)
• Object is exported to disk as PLY and converted to a low-resolution proxy object in Blender

Implementation:

• The proxy has to support multiple materials. Back in LuxBlend we split it into multiple objects with one material each and parented them to one object, but this is far from ideal (user can accidentally move a part, and it is clumsy to use). A better idea:
  • Only one proxy object in Blender
  • Has properties attached with information about the multiple PLY files attached to the proxy and which material belongs to which PLY
• We will probably need to add a function to pyluxcoreforblender that exports a Blender mesh to PLY because the PLY exporter in Blender is written in Python and super slow.

Details:

• The preview mesh should have equal density over its volume (not depending on the original mesh density). Otherwise you get the problem that e.g. the preview mesh of a tree with highpoly leaves and a lowpoly trunk has almost no density around the trunk. (forum post)

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