Improve behaviour of the 'cordon' button

[bboreham]

After #3822 the merging of active controls is still a bit off - when you press 'cordon' it doesn't update for a few seconds, then you get both cordon and uncordon buttons, then a bit later the cordon button goes away.

I believe the slow reaction is because the display is updated on a 5-second timer; we could perhaps use a 'shortcut report' to speed it up.

Displaying both sides of the toggle button happens because the app merges reports from ~15 seconds of history, during which time both buttons appear in reports from the probe. Given we have special merging code for this field, maybe we can have it recognize the toggle nature.