Validate targets if widget kind is 'graph'

The example in the API.md doesn't work

```sh
curl -v -H "Content-type: application/json" -X POST -d '{ "name": "test", "source": "demo" }' http://localhost:3000/api/dashboards/1/widgets
```

creates an invalid graph widget because of missing targets.

I added a validation of 'targets' to the widget model in case the widget is of kind 'graph' and adapted the API.md