[2023] Fix MenuItem label sorting (of procs)

An alternative approach to #2023.

@macfanatic's changes to MenuItems in 452956ec changed label procs to
be evaluated in the view context, but this broke MenuItem sorting. This
commit moves the sorting logic up into TabbedNavigation, where the view
context is accessible.

I don't at all like this solution -- it's not at all OO -- but at least
it fixes the regression.