Selfd countdown polishing

Right now selfd has issues:
* hardcodes console messages for the countdown
* hardcodes the assumption that gamespeed == unit slowupdate speed * 2
* is unreliable, i.e. a zero countdown still waits for the next slowupdate
* doesnt really let the game know about the countdown.

Fix those!
* count down per frame and not per slowupdate.
* make sure games can input arbitrary timeouts given the above
* in slowupdate, send a lua event with remaining time and event period, in seconds. Send an event on start and cancel as well.
* dont send console messages. Provide a trivial replacement widget that provides equivalent messages to the existing ones.

Start by locating selfd and its slowupdate path, then trace how countdowns start, cancel, and currently produce console messages. Done means frame-based arbitrary timeouts, Lua events carrying remaining time and event period in seconds on start, cancellation, and slowupdate, plus a widget replacing the existing messages.

Written by the indexing model from the issue text.