#mtlLibPreOpen: undefined Sent just before loading a material library.
#mtlLibPostOpen: materiallib Sent just after loading a material library.
Calling callbacks.notificationParam() within the callback function will return the MaterialLibrary loaded.
#mtlLibPreSave: undefined Sent just before saving a material library.
#mtlLibPostSave: undefined Sent just after saving a material library.
#mtlLibPreMerge: undefined Sent just before merging a material library.
#mtlLibPostMerge: undefined Sent just after merging a material library.