Changes to how you add shaders in the UI

[NMC-TBone]

• The shaders will now be automatically loaded in a EnumProperty through the data path set in addon preferences
• This includes a operator that will migrate the old "source" property over to the new "shader" enum property (and preserve all the other shader related data)
• Made it possible to add "material_name" to the load_custom_shader and load_custom_shader_variation operators so it can be called through API without needing to have a active object with a active material to run the operators

Should hopefully fix #179

[ssnd292]

Guess this would also fix https://github.com/StjerneIdioten/I3D-Blender-Addon/issues/170