QML: Use Pane as container in settings example

Using QtQuick.Controls Pane allows the resulting UI element to be styled
by system theme.
Using Rectangle forces some color, if not specified white is used.

Signed-off-by: Juraj Oravec <jurajoravec@mailo.com>

qml/articles/8.Extension_settings.md

@@ -28,12 +28,14 @@ X-Falkon-Settings=true
 
 main.qml
 ```qml
+import QtQuick.Controls 2.3
+
 Falkon.Settings {
     id: settings
     name: 'Tutorial8_settings'
 }
 
-settingsWindow: Rectangle {
+settingsWindow: Pane {
     id: window
     width: 256
     height: 200

qml/extensions/qml_tutorial_8/main.qml

@@ -28,16 +28,17 @@ Falkon.PluginInterface {
         toolTip: i18n('My little button')
         icon: 'falkon'
         location: Falkon.BrowserAction.NavigationToolBar | (settings.value({key: 'statusbar', defaultValue: 1}) == 1 ? Falkon.BrowserAction.StatusBar : 0)
-        popup: Rectangle {
+        popup: Pane {
             width: 100;
             height: 100;
         }
     }
 
-    settingsWindow: Rectangle {
+    settingsWindow: Pane {
         id: window
         width: 256
         height: 200
+
         Image {
             id: image
             source: 'qrc:/icons/other/about.svg'
@@ -48,15 +49,18 @@ Falkon.PluginInterface {
         CheckBox {
             id: checkStatusbar
             checked: settings.value({key: 'statusbar', defaultValue: 1}) == 1
-            text: 'Show in statusbar'
+            text: i18n('Show in statusbar')
             anchors.top: image.bottom
         }
         Button {
             id: button
             text: i18n('Save')
-            width: 256
             height: 50
+
             anchors.top: checkStatusbar.bottom
+            anchors.left: parent.left
+            anchors.right: parent.right
+
             onClicked: function() {
                 var res = settings.setValue({
                     key: 'statusbar',