Use a toolbar instead of a header for the installed page

BUG: 388720

discover/qml/InstalledPage.qml

 import QtQuick 2.1
-import QtQuick.Controls 1.1
+import QtQuick.Controls 2.1
 import QtQuick.Layouts 1.1
 import org.kde.discover 2.0
 import org.kde.discover.app 1.0
@@ -16,7 +16,14 @@ ApplicationsListPage {
     compact: true
     canNavigate: false
 
-    listHeader: Kirigami.ItemViewHeader {
-        maximumHeight: minimumHeight
+    listHeader: null
+    header: ToolBar {
+        Kirigami.Heading {
+            anchors {
+                right: parent.right
+                rightMargin: Kirigami.Units.smallSpacing
+            }
+            text: page.title
+        }
     }
 }