Widget graph set duration

Hi !

This pull request add the ability to select the duration of a dashboard graph widget.

This is mainly usefull if a service you want to graph on dashboard have a check intervale grather than one hour.

In order to do that i've modify the widget select input template. We can now specify a select element more easily.

```
        'duration': {
            'value': 'id of selected value',
            'values': {
                'Label Option 1': 'id option 1',
                'Label Option 2': 'id option 2',
                'Label Option 3': 'id option 3',
            },
            'type': 'select',
            'label': 'Global label'
        },
```

Hope you'll like this feature ;).

regards