feat(core/canvas): add generic element.changed event
feat(core/canvas): add generic element.changed event

Related to #39

thr0w/diagram-js,pedesen/diagram-js,chudaol/diagram-js,bpmn-io/diagram-js,tsneed290/diagram-js,thr0w/diagram-js,tsneed290/diagram-js,pedesen/diagram-js