implement drag/drop for PowerManager and ScreenSaver actions

see https://github.com/lxde/lxde-qt/issues/319
The PowerManager actions have their identifier stored as value "action" in the settings when added to the quicklauncher.
So when we have the "action" value filled it's a PowerManager action. We then loop over "otheraction" and when we find the action identifier, an appropriate QuickLaunchAction is added.
I just see that the "bfound" variable is not needed here and may be a bit misleading...