Keep an operator's windows, colours and recordings

Ctrl+O asks who is at the radio, as it does in N1MM, and the callsign goes on
every contact logged from then on. A multi-operator station changes hands every
few hours, so each operator also keeps what he wants the program to look like:
where the windows sit, the theme, and the folder his recordings are in.

The windows on the screen are stored against the operator leaving, before the
dialog opens, and the operator taking over gets his own back. A window he opens
later is placed as it appears. An operator who is new to the list starts with
whatever is on the screen.

Nothing plays the recordings yet, so the folder is stored and waits on voice
keying. {OPERATOR} in a message sends the callsign.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RoGtneMQaz4M9w7Kk49AVD

docs/unfinished.md

@@ -35,7 +35,8 @@ The other interfaces N1MM offers — fldigi, MMVARI, the hardware TNCs — are n
 here either, though `DigitalEngine` is the place to add them.
 
 **Voice keying.** `MessageSender` was written to cover a voice keyer playing a
-recording, and nothing implements it. No DVK support either, so the second radio
+recording, and nothing implements it. Each operator's recordings folder is
+stored with the rest of his settings and waits on the same thing. No DVK support either, so the second radio
 of an SO2R station cannot call CQ by voice. Alternating CQ therefore works on CW
 only, though nothing in it is CW-specific: a voice keyer that reports when the
 recording has finished would drive it as it stands.