POV: fix some nested code and further files structure cleanup

* FIX: wrongly nested pov braces made the default outpout file fail
* FIX: use agnostic metallic property rather than create a duplicate
* FIX: some 2.8 deprecated properties rewired in spec;diff; emit;ambient
* FIX: clean up, hierarchize and redesign Global Settings ui panel
* FIX: re-wire world background alpha to agnostic prop and redo its ui
* FIX: wrong nested pov braces making the default outpout file fail
* FIX: use agnostic metallic property rather than create a duplicate
* FIX: reduced arguments numbers by imports and relocating variables
* FIX: use more list comprehesions to reduce nested conditions levels
* FIX: use more consistent class names but cleanup still not finished
* FIX: use single quotes for enums preferably to distinguish strings
* FIX: basic level of nodes based material (diffuse color) broken API
* FIX: blurry reflection corner case caused output file to fail
* FIX: added context managing ("with") syntaxes reducing crash cases
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* ADD: model_all.py file to extract mostly object level loop and utils
* ADD: model_meta_topology.py file to extract metaballs export
* ADD: object_primitives_topology.py to extract pov compound primitives
* ADD: nodes_fn.py file to extract main node exporting function
* ADD: nodes_gui.py file to extract node operators and menus
* ADD: nodes_properties.py file to extract nodes sub parameters
* ADD: particles_properties.py to extract particles and fx parameters
* ADD: render_core.py to extract main RenderEngine inheriting class(es)
* ADD: shading_ray_properties.py to extract pathtraced shader parameters
* ADD: texturing_procedural.py to extract algorithmic texture influences
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* UPDATE: workspace tools icons and a couple of other icons choices
* RENAME: pov.add.polygontocircle.dat macro workspace tool icon
* RENAME: base_ui.py to ui_core.py
* RENAME: shading_nodes.py to nodes.py
* RENAME: df3_library.py to voxel_lib.py to make dot lookup inform more
* RENAME: object_mesh_topology.py to model_poly_topology.py
* RENAME: object_curve_topology.py to model_curve_topology.py
* RENAME: object_gui.py to model_gui.py
* RENAME: object_primitives.py to model_primitives.py
* RENAME: object_properties.py to model_properties.py
* RENAME: object_particles.py to particles.py