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/Diary.md

@@ -13,7 +13,9 @@ an entry written after midnight on a trip still lands on the right day.
 
 ## The calendar
 
-`C` (or `:cal`) shows the diary a month at a time.
+`C` (or `:cal`) shows the diary a month at a time, in the
+[special workspace](Boards-and-Agenda.md#the-special-workspace) under the
+agenda.
 
 ![The diary by month](../images/calendar.png)
 
@@ -24,7 +26,7 @@ an entry written after midnight on a trip still lands on the right day.
 | `H` / `L` | a month back / forward |
 | `.` | today |
 | `Enter` | open the day's entry, or start one dated that day |
-| `C`, `Esc` | close |
+| `C`, `Esc` | put the special workspace away |
 
 A dot marks a day with an entry. A number says how many when there are more
 than one.