wallpapers/image: add opacity animation in wallpaper item

wallpapers/image/imagepackage/contents/ui/WallpaperDelegate.qml

@@ -5,7 +5,7 @@
     SPDX-License-Identifier: GPL-2.0-or-later
 */
 
-import QtQuick 2.0
+import QtQuick 2.15
 import QtQuick.Controls.Private 1.0
 import QtQuick.Controls 2.3 as QtControls2
 import QtGraphicalEffects 1.0
@@ -124,6 +124,13 @@ KCM.GridDelegate {
         }
     }
 
+    Behavior on opacity {
+        OpacityAnimator {
+            duration: PlasmaCore.Units.longDuration
+            easing.type: Easing.InOutQuad
+        }
+    }
+
     onClicked: {
         if (configDialog.currentWallpaper == "org.kde.image") {
             cfg_Image = model.packageName || model.path;