Allow to set custom ViewController title

Some people like me want the AcknowledgementsViewController to have a custom title like "Licenses" or "Software Licenses" (or custom localized versions thereof) instead of "Acknowledgements.

By setting the `title` to the `localizedTitle` in the common initializer the functionaloty is unchanged, but developers are able to set `UIViewController`'s `title` property after initialization:

```
VTAcknowledgementsViewController *licensesViewController = [VTAcknowledgementsViewController acknowledgementsViewController];
licensesViewController.title = NSLocalizedString(@"my_custom_licenses_title", nil);
[self.navigationController pushViewController:licensesViewController animated:YES];
````