Enable/Disable the toolbar buttons according to what the user has selected

If the user hasn't selected any download, LottaNZB should disable the "Move xy" and "Remove" buttons to make it clear that a click on one of these buttons won't have any effect. We could do the same for finished downloads.

And of course is it impossible to move down a download that is at the very bottom of the queue. LottaNZB should detect this as well. The top of the queue should be handled analogue.