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

Over here you do unnecessary work there as there is only TextField or TextSwitch visible. 

Rule of thumb: do as little work as possible in delegate construction.

You have 16ms per each frame and frame can contain multiple new delegate instances created by the listview. Thus, let's remove the cost of creating two childs (TextField is also rather expensive component).

Column to Loader
```
delegate: Loader {
    sourceComponent: model.type === 128 ? textSwitch : textField
```