Show advanced settings when 'about:config' url is visited

cache read returned value and get(i) only once.
```
var prefJson = prefsListModel.get(i)
if (prefJson.name.toLowerCase().search(value.toLowerCase()) != -1) {
    filterListModel.append(prefJson)
```

Also resetting model is something that we should try to avoid when ever possible (as all delegates will be recreated). In this case reset is ok. In normal use removing and adding rows is better.