Merge tag '4.9.3' into develop

Features:
* Added remove bone geometry tool and fixed minor glitch with add bone geometry tool
* Error handling improved.
* It works on wine (Linux/Mac OS).

Fixes:
* Normal maps are now properly exported into mtl file.
* Fixed bug if image path is empty
[#6] Import organising, added self in several places.
[#6] Import organising, added self in several places.
[#6] extracted __export_library_controllers from export
[#6] Deleted commented out code.
[#6] Extracted doc as class privete field.
[#6] export function splitted up. Fixed up 'self' param issue.
[#6] Extracted __export_library_visual_scenes from export()
[#6] Code cleaning, veriable renames.
[#6] Extracted __export_library_animation_clips_and_animations from export.
[#6] Extracted __export_library_effects and __export_library_materials
[#6] Exctracted __export_library_geometries and __export_scene
[#6] God export method splitted up. Small bugfixes.
[#6] Code cleanning and refactoring.
[#6] Deleted commented out code, unused variables, small refactoring.
[#6] Removed unecessary "if" statement
[#6] Removed unnecessary len().
[#6] variables renamed, removed statements without effect.
[#6] Methods moved to match call order.
Changed file type selection to drop-down menu