Patch to review: desktop icon overlap

[Bill Smith <snowmanam2 gmail com>]

At the risk of posting another patch that may be inadequate, here's
another attempt at fixing the desktop icon overlap. 

This time, for moving icons, I used find_empty_location() and a
PlacementGrid to adjust icons without moving others. This would replace
snap_position() in nautilus_icon_container_move_icon(). For adding
icons, the placing code already exists in finish_adding_icons(), but it
was not being used (according to my testing). I'm not sure if the way
I'm proposing fixing this is best (setting has_lazy_position if we're in
keep_aligned mode), so any other ideas are appreciated. 

Thanks,
Bill