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.