[KSplash] Start with zero opacity

The `introAnimation` only kicks in at stage 2 so you might catch a
glimpse of the logo before its fade-in animation starts.


(cherry picked from commit 5f8f1514)

lookandfeel/contents/splash/Splash.qml

@@ -28,6 +28,7 @@ Rectangle {
     Item {
         id: content
         anchors.fill: parent
+        opacity: 0
 
         Image {
             id: logo