Material viewport mode (aka LookDev)

This issue is about implementing a custom viewport mode in BlendLuxCore for Blender's "material" shading mode (the third shading mode, besides wire/solid/rendered). It should offer a quick and noise-free preview of the most important textures of a material node tree.

Currently, it uses an RTPATHCPU engine with simplified settings and shows the Albedo AOV in the viewport.
Video: https://youtu.be/mzxQaxemWHI

Ideas:

• Skip all non-relevant textures during export (will require heavy changes to the internal node export API). Not sure if this is necessary.
• Add emission AOV (for background HDRI, and preview of emissive materials)
• Option to define only the materials on the current object, use dark grey matte for all others (to speed up initial export with many and/or high-res textures in the scene)
• Keyboard shortcuts like in node wrangler to preview the selected texture in the material (e.g. set as diffuse color of a matte material, or as emissive color) (added as Ctrl+Shift+T in #407)
• Prevent noise from arch. glass and homo/heterogeneous volumes (disable reflections? replace with clear volumes?)

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