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

Missing qsTrId from the the text.
```
//: Warning of changing browser configurations.
//% "Changing these settings may void your warranty. Continue?"
var dialog = pageStack.push(Qt.resolvedUrl("components/ConfirmDialog.qml"), {text: qsTrId("sailfish_browser-la-config-warning")});
```
Need to think what should be said here. FF warning like: "Changing these advanced setting can be harmful to stability, security, and performance of this application. You should continue only if you are sure of what you are doing?"

Better would be to refer to ... "performance of the Sailfish Browser."