Utilities for refactored Blender add-on

[Speierers]

This PR contributes necessary updates and utilities for the major refactoring of the Mitsuba-Blender add-on.

Proposed changes:

• The MeshHolder shape plugin, a wrapper around Mesh to hold a mesh instance in a Python dictionary using tensors.
• Improvements to the blender shape plugin.
• Improvements to the mi.xml_to_props routine for importing Mitsuba scenes in Blender.
• Introduction of the mitsuba.blender submodule, that implements performance critial routines in C++, exposed in Python to be used by the add-on.
• The Mesh constructor now takes an optional displacement_map texture nested object that's used during construction to displace the vertex positions.