There was no convenient way for an add-on to show it's "user"
key-map items in the preferences, "user" meaning the key-map
item which is edited in the key-map editor, not the original
key-map item created in `wm.keyconfigs.addon`.
Add a method to key-maps to lookup a user key-map item from an
add-on key-map & key-map-item.
Ref !134830
4a6d687d53