QML: Switch to Pane for sidebar tutorial

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

qml/articles/7.Adding_a_sidebar.md

@@ -5,16 +5,18 @@ Hello, in this chapter I will show you how to create a custom sidebar.
 ## Create sidebar
 
 ```qml
-    Falkon.SideBar {
+import QtQuick.Controls 2.3
+
+Falkon.SideBar {
     name: 'qml-tutorial7-sidebar'
     title: i18n('Tutorial 7 - QML SideBar')
     icon: 'falkon'
     checkable: false
     shortcut: 'Ctrl+Shift+Alt+S'
-        Rectangle {
+    Pane {
         /* Actual UI and some drawing logic */
     }
-    }
+}
 ```
 
 ### Falkon.SideBar

qml/extensions/qml_tutorial_7/main.qml

@@ -23,7 +23,8 @@ Falkon.PluginInterface {
         icon: 'falkon'
         checkable: true
         shortcut: 'Ctrl+Shift+Alt+S'
-        Rectangle {
+
+        Pane {
             Image {
                 source: 'qrc:/icons/other/startpage.svg'
                 anchors.verticalCenter: parent.verticalCenter