Fix Blur behind in Dashboard Effect

By making Dashboard Effect a fullscreen effect the blurring got
accidentially broken (blur ignores fullscreen effects). Reenabling
by setting the force blur role on the dashboard window(s).

Reapplied commit 629e17d18526ccfae3d3579066e8be927a92437d from KDE/4.6.

CCBUG: 259797

effects/dashboard/dashboard.cpp

@@ -164,6 +164,7 @@ void DashboardEffect::slotWindowActivated(EffectWindow *w)
 
         if (blur) {
             w->setData(WindowBlurBehindRole, w->geometry());
+            w->setData(WindowForceBlurRole, QVariant(true));
         }
 
         effects->addRepaintFull();