Improve handling of material blending modes in the Blender add-on

[davidrios]

This PR improves the handling of materials from m2 and wmo objects when importing using the Blender add-on, so also fixes #392.

It:

• creates different materials for different blending modes
• avoids creating materials and material slots on objects for textures which aren't being used
• creates emissive materials for certain objects to improve rendering, with an option to disable if needed

Comparisons (before/after):

[rjallanreal]

@Kruithne Any input on this pull request? It looks like a marked improvement to me.

[BSarmady]

Kudos, Great work. Thank you.

Does this new version support new models too?