notmuch.el: Hook up support for different search orders.

This gives us two different default search orders: The inbox view that
comes up from "M-x notmuch" is sorted in chronological order (oldest
threads first). A new global search "M-x notmuch-search" will instead
be in reverse chronological order (newest threads first).

Any filtered searches retain the sort order of the search being
filtered.

There's not yet any interface for changing the sort order of a search
after it is constructed.