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