WidgetExplorer: Only count applets in visible Containments

Currently the applet count is filtered by activity. But for every activity all applets in all known screens/containments are counted. If a user switches screens regularly they will see for example that there are 7 instances of the clock applet even though they have less screens and only one instance in their current panel(s). This information is not useful to the user because they cannot interact with the not visible applets in any way.