I'd like the desktop to be a virtual grid where
icons can snap. For example, moving the Home icon
by 5 pixels shouldn't move it at all. But moving
it by 20 pixels should move it to the next grid case.

I think that can be easily done by simply dividing
each icon's coordinates by - say - 50, getting an
integer (not a float), and mutliplicating this
integer by 50. Of course the size of the cases
should be smartly decided to avoid icons
overlapping other icons.