Add settable callback on change from backend
Add settable callback on change from backend

If settings are updated a callback can be set to notify the skill that a
change has occured.

forslund/mycroft-core,Dark5ide/mycroft-core,linuxipho/mycroft-core,MycroftAI/mycroft-core,forslund/mycroft-core,linuxipho/mycroft-core,MycroftAI/mycroft-core,Dark5ide/mycroft-core