This method is called after a snip in the pasteboard is selected or deselected. See also on-select. This method is not called after selected snip is deleted (and thus de-selected indirectly); see also after-delete.
See also can-select? and on-edit-sequence.
No internals locks are set when this method is called.
If on? is #t, then snip was just selected, otherwise snip was just deselected.