emacs: Add new customization option to sort saved searches

Add new customization option notmuch-saved-search-sort-function to sort
saved searches in user-defined order. Provide a sort function to sort the
saved searches in alphabetical order. Setting the search function to nil
causes the saved searches not to be sorted, as before. This also remains
the default. The function only affects display of the saved searches, not
the order in which they are stored by custom.

Signed-off-by: Jani Nikula <jani@nikula.org>