panel: Raise BusyIndicator above everything else

That's what ContainmentLayoutManager.AppletContainer is doing in C++.

(cherry picked from commit ce657881)

containments/panel/contents/ui/main.qml

@@ -282,7 +282,9 @@ function checkLastSpacer() {
             }
 
             active: applet && applet.busy
-            sourceComponent: PlasmaComponents.BusyIndicator {}
+            sourceComponent: PlasmaComponents.BusyIndicator {
+                z: 999
+            }
 
             property int oldX: 0
             property int oldY: 0