Added handling for group's getHelperText element in backstage view

This commit adds support for binding getHelperText attribute in group element for Backstage view

```xml
 <backstage>
    <tab id="MyBackstageTab" label="Backstage" >
      <firstColumn>
        <group id="Settings" label="Addin settings" getHelperText="GetSettingsHelperText" />
      </firstColumn>
    </tab>
</backstage>
```