special: the board, the agenda and the calendar in a workspace of their own

The board swapped out the tree, and the agenda and the calendar were
overlays over it, each with a key to open and a key to close. They are now
three tiles of one special workspace, as the desktop keeps its scratchpad:
the board on the left, the agenda over the calendar on the right.

S puts it up and away from anywhere. b, t and C go straight to their tile,
and put the workspace away when that tile already has focus. Alt-h/j/k/l
move the focus between the tiles, f gives the focused one the porthole,
and the tiles without focus keep their cursors, dimmed. A digit switches
space as everywhere, and the board follows; the board's head names every
space that has one. Enter on an agenda card goes to it on the board beside.
A space without a board says so in the board's tile instead of refusing.

The rail offers the three tiles as doors, so a phone, which shows one at a
time, can walk between them. The help, the README, the wiki and the
screenshots say how it works now.

docs/wiki/Getting-Started.md

@@ -72,7 +72,8 @@ The rail of buttons on the right does the same as the keys, for a mouse.
    `h` comes back up.
 6. Press `c`, type `try the board`, and press `Enter`. That is a card in the
    inbox of this space's board. Press `b` to see the board, and `b` again to go
-   back.
+   back. It shares a special workspace with the agenda and the calendar: `S`
+   puts it up, and `Alt-h`/`Alt-l` move between them.
 7. Press `T`. That is today's diary entry, started for you.
 8. Press `?` for the list of every key and command. `h` / `l` walk its columns, `/` narrows it down, and `Esc` closes it.