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

We should have means to reset preferences. PullDown menu would be natural choice.

PullDown menu item could look like:
```
            MenuItem {
                //: Reset browser configuration to default values
                //% "Reset to default"
                text: qsTrId("sailfish_browser-me-reset-configuration_to_default")
                onClicked: ... // do the work e.g by removing from mozembed dir prefs.js
            }
            MenuLabel {
                //: Menu item info label so guiding to restart browser after reset.
                //% "Restart browser after reset"
                text: qsTrId("sailfish_browser-me-restart-browser")
            }
```