Update some files to QQC2 and fix minor bug

Summary: The bug was that the Drawer use the `drawerOpen` properties now instead of just `open`. Also fix deprecation Kirigami.Label -> QQC2.Label.

Test Plan: Tested on my computer with the touchpad. I could potentialy try with my android phone, if needed.

Subscribers: okular-devel

Tags: #okular

Differential Revision: https://phabricator.kde.org/D18637

mobile/app/package/contents/ui/Thumbnails.qml

@@ -41,7 +41,7 @@ ThumbnailsBase {
                 }
             }
         }
-        Kirigami.Label {
+        Label {
             anchors {
                 left: searchField.right
                 verticalCenter: searchField.verticalCenter

mobile/app/package/contents/ui/ThumbnailsBase.qml

@@ -82,7 +82,7 @@ Kirigami.Page {
                                 bottom: parent.bottom
                                 right: parent.right
                             }
-                            Kirigami.Label {
+                            Label {
                                 text: modelData + 1
                             }
                         }

mobile/app/package/contents/ui/TreeDelegate.qml

@@ -18,6 +18,7 @@
  */
 
 import QtQuick 2.1
+import QtQuick.Controls 2.0
 import org.kde.kquickcontrolsaddons 2.0
 import org.kde.kirigami 2.0 as Kirigami
 
@@ -45,7 +46,7 @@ Column {
         onClicked: {
             documentItem.currentPage = page-1
 
-            contextDrawer.opened = false
+            contextDrawer.drawerOpen = false
         }
 
         QIconItem {
@@ -56,7 +57,7 @@ Column {
             anchors.verticalCenter: parent.verticalCenter
             x: units.largeSpacing
         }
-        Kirigami.Label {
+        Label {
             id: label
             text: display
             verticalAlignment: Text.AlignBottom
@@ -73,7 +74,7 @@ Column {
                 right: pageNumber.left
             }
         }
-        Kirigami.Label {
+        Label {
             id: pageNumber
             text: pageLabel ? pageLabel : page
             anchors.right: parent.right