Limit search time

Currently, you set the AI strength by iteration count. Change it to limit by search time. Also change AI strength plot to use search time. Add average iterations as extra information?

* [x] Add options to limit by iterations or milliseconds.
* [x] Update game display.
* [ ] Update strength measurement display.

Start by locating the current iteration-based AI strength setting, the game display updated by this issue, and the strength measurement plot. Trace how the completed iteration-or-milliseconds options flow into those displays. Done means the remaining strength measurement display uses search time, with average iterations included if the existing design supports it.

Written by the indexing model from the issue text.