Add: new ManualMotionDispatcher

[yn01-dev]

Add a new implementation of ManualMotionDispatcher.

In v1, ManualMotionDispatcher was a static class, but in v2, it is a normal class and can be instantiated.

var dispatcher = new ManualMotionDispatcher();

LMotion.Create(0f, 1f, 1f)
    .WithScheduler(dispatcher.Scheduler)
    .Bind(x => { });

dispatcher.Update(0.5);
dispatcher.Time = 1.0;

Also, ManualMotionDispatcher.Default is available as a global dispatcher. Also, the implementation of MotionScheduler.Manual is changed to be the same as ManualMotionDispatcher.Default.Scheduler.