Added limit attribute for the directory provider

This adds the ability to limit directory content to a certain amount of items.

```
<content target="videos" limit="10">videodb://movies/titles</content>
```

Default value is 0 which will show all available items.

PS: Would be nice to have in Gotham but +1 is also fine.
It's a little bit better to read in the code in my opinion and even if the conversion is easy, you don't need to bother with it at other places.