Add a grace period between detecting a change and triggering generation in live preview

[modelflat]

As per discussion in #1248 this PR adds logic to delay the live preview generation upon change (without the extensions mentioned in this comment).

[Acly]

This is more of an absolute timer though, which doesn't refresh if there are more changes after the first change, right?
I understood it as a (fairly short) period where there are no more changes instead.

It might be easier to implement that by tracking the timestamp of the last change. Since it's already a loop with a sleep it could be quite simple, something like

if self._last_input != new_input:
    current_time = time.monotonic()
    if current_time - self._last_change < grace_period:
        await self._model._generate_live(new_input)
        self._last_input = new_input
    self._last_change = time.monotonic()

I think this would make it unnecessary to pass _last_input to _generate_live and call _prepare there since that part is already handled. The just_got_here would also not be needed, since the _last_change timestamp would naturally be quite old in this case.