popup menu
Extends append-editor-operation-menu-items so that when you
pass an editor and a position, it checks to see if that spot
has a non-string% snip and, if so, copies that one position
(or cuts it, depending).
Then, use that extension in DrRacket
closes PR 12791
original commit: 6fff8a3030bfaddd0eb8113c9f67d6b7e70500ff