Use KActionCollection::addAction instead of add<T>

I'm not sure for what reason this was changed but it is clearly wrong,
we don't want to create a new action but rather re-add (as the hack says)
the action we took before.

Fixes "Empty action" in toolbox.

src/plasma/private/applet_p.cpp

@@ -250,7 +250,7 @@ void AppletPrivate::updateShortcuts()
         for (int i = 0; i < names.size(); ++i) {
             QAction *a = qactions.at(i);
             if (a) {
-                actions->add<QAction>(names.at(i), a);
+                actions->addAction(names.at(i), a);
             }
         }
     } else {