Fixed onAdd/onUpdate callbacks not working when using DataView in Timeline

onAdd/onUpdate callbacks throw an error when using DataView in Timeline.

```
Uncaught TypeError: undefined is not a function vis.js:12312
(anonymous function) vis.js:12312
ItemSet.defaultOptions.onAdd vis.js:10981
ItemSet._onAddItem vis.js:12310
triggerEvent vis.js:27847
tapGesture vis.js:28366
triggerGesture vis.js:27556
each vis.js:26782
detect vis.js:27553
doDetect vis.js:27265
onTouchHandler vis.js:27184
```

It looks like a handler tries to call add()/update() functions directly on DataView object, which doesn't have ones.