agenda: a card's date and list without going to its board

In the agenda, `D` asks for the highlighted card's due date on the command
line (`:due` now works there as well as on the board), and `H` / `L` send
the card to the list left or right of the one it is in, on its own board.
`L` into the last list is done, as on the board.

The command line now rises above an open overlay, and while it is up the
overlay's own keys stay out of its way, so typing a date does not walk the
agenda.

README.md

@@ -230,7 +230,8 @@ file is refused.
 due within the week, or sitting in a column between the inbox and done — the
 work already in hand — and says whether the diary has an entry for today. `j`/`k`
 walk it, `Enter` goes to the card on its own board, `Space` finishes it where
-it stands, and `T` writes today's entry. The bar shows `late` and `today` counts
+it stands, `D` sets its due date (`:due` on the line), `H` / `L` send it to
+the list left / right on its board, and `T` writes today's entry. The bar shows `late` and `today` counts
 whenever either is above zero, and the agenda opens by itself once a day when
 something is late or due, so nothing has to be remembered to be looked at.