Make `keymap-set-after' work for menus

Make `keymap-set-after' work for menus

It still doesn't work for an AFTER that's a key, though, since
`key-parse' produces vectors, and keymaps contain integers.

* lisp/keymap.el (keymap-set-after): Only parse AFTER as a key if it's
a string.  For consistency, use `key-parse' on the definition if it's
a string, just like `keymap-set'.
* test/src/keymap-tests.el (keymap-tests--command-3): New dummy command.
(keymap-set-after-menus): New test.  Check that we can insert a menu
item after a specific entry.