add IsQuitting boolean, Quitting event, and StopOnQuit helper

[GeBo1]

Add support to detect/react to application quitting.

Plugins with long waits can cause game/studio to hang when quitting. Replace things like:

yield return new WaitForSecondsRealtime(ConfigSetting.Value * 60);

with:

yield return new WaitForSecondsRealtime(ConfigSetting.Value * 60).StopOnQuit();
if (KoikatsuAPI.IsQuitting) yield break;

and/or:

KoikatsuAPI.Instance.Quitting += (s,e) => StopAllCoroutines();

[ManlyMarco]

I'm not sure about the coroutines, the event itself seems fine.

Are you sure the coroutines reliably get a chance to run after quitting starts?

If some coroutine needs to handle game quitting then it can add the condition by itself, having extra classes for this seems like an overkill with how niche this is.

[GeBo1]

I've done experiments and found that (at least in some cases) they continue running and the game waits for them to complete. I've updated most of my plugins to watch for quitting, but I'm pretty sure autosave is hanging studio shutdown with it's wait for X minutes (which is why I wanted to move this to someplace more common), so I figured something more generic rather than everyone writing similar code, but I can remove them.

The other option is to just not use the WaitFor at all and do something like this:

var timeout  = Time.time + delay;
while (Time.time < timeout)  
{
    yield return null;
    if (IsQuitting) yield break;
}

[GeBo1]

Removed extra classes and added single StopOnQuit() extension method.