[kwin] Make desktop change OSD a click-through window

It used to be like that, but regressed with the switch to QML. Now
PlasmaCore.Dialog supports an outputOnly mode, so let's add it again.

scripts/desktopchangeosd/contents/ui/osd.qml

@@ -28,6 +28,7 @@ PlasmaCore.Dialog {
     location: PlasmaCore.Types.Floating
     visible: false
     flags: Qt.X11BypassWindowManagerHint
+    outputOnly: true
 
     mainItem: Item {
         function loadConfig() {