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