applets/notifications: Notification positioning fixes
Better tracking of obstructingDialog size, visibility and position, making notifications reposition correctly when going out of edit mode
Behavior is still a bit weird sometimes, but should never be breaking anymore, for a full fix the obstructingDialog should be rewritten considering that there might be more than one, but this would lead to a considerable increase in code complexity, so doesn't belong in this mr.
More robust tracking of who is the parent plasmoid: the hack used before alsways removed the first item from plasmoidItems which is not guaranteed to actually be the one that was removed, now plasmoids unsubscribe themselves before actually being destroyed, which is much more reliable