Fixesatom/atom-keymap#79
This is more general than I would like. If the keystroke is unset in
any context, we err on the side of caution and don’t add it to the
application menu for any command. Since our application menu isn’t
context aware, this should be good enough for now and solve the 80%
case. Someday we should make the application menu update / gray out
options when the focused element changes.