inspired by beat wolf's patch on review board, this makes the device notifier behave a lot better on the desktop (still proxy widget issues to work out, but nothing beyond the ordinary) and now dragging between desktop and panel work properly.

it's a slightly easier and more straightforward approach than what was there before (no "isOnDesktop", which really doesn't make a lot of sense unless one assumes that plasma is a desktop, which is not a safe assumption) and once that was straightened out, beat wolf's patch becomes a lot easier to implement straight forward ...