Move lag compensation and clock improvements

[veloce]

Make the real time game clocks more precise by implementing:

A new chess clock for playable games with:

• server lag compensation (network lag returned by server)

An update of CountdownClock widget:

• UI lag compensation (time between the received clock event and when the change is displayed)
• server lag compensation (for TV so it matches server, even though it is less important to have a precise time here)

The latter is no more used for playable game, only for read-only display of clocks where live clock ticking is needed (TV, broadcast, etc).

[julien4215]

Why are we using this package for the countdown clock widget ?

I don't see why Datetime.now and the builtin Stopwatch would not work.

[veloce]

To be able to test the clocks. Contrary to DateTime, clock is overridden in fakeAsync environment. So we can reliably measure the elapsed time in tests.