restrict editing in areas that are not downloaded

[dieterdreist]

1. JOSM prohibits deletion of nodes in areas where you haven't downloaded. But it still permits moving and adding nodes in those off-areas. IMHO it would be better to prohibit any editing in those areas.
2. When you try to delete there is currently no user-feedback. The item get's unselected and not deleted, but there should be an additional alert explaining why it didn't work (and if possible also an accoustic alarm): "Cannot delete in areas that are not downloaded" or similar.