In default KDE panel use Show Desktop instead of Show Dashboard widget

We should use the Show Desktop widget instead of Show Dashboard. The problem is that some users are confusing Show Dashboard with the Show Desktop functionality like in Windows. The widgets have similar but different functionality and it seems like more people want Show Desktop than Show Dashboard (probably because more people are familiar with the Windows function than the dashboard concept. We don’t configure multiple activities by default in Kubuntu (yet) and so the Dashboard concept hasn’t been really pushed on non-KDE-power-users yet. Having both on the panel by default is problematic because they look very similar.