This helps avoid a situation where someone connects to the change signal for
PREFIX but really wants PREFIX_MODES.

This is a work around for the lack of a freeze like ability for features which
is why these were able to get out of sync before this. Technically they're
still out of sync and that needs to be fixed, but this should cover the
majority of use cases.

The additional unit tests don't test for this bug, but were added while
debugging and seem useful.

Called in the turtles and verified the code for another review request worked with these changes.