Release v5.7.0 - #43

Sprites load their costumes without a window, so sizes, costume switching,
hitboxes and clones can be checked in plain unit tests. Without a window,
timers stand at 0.

Variable monitors, like Scratch's "show variable": showVariable("score", () -> score) on a stage or a sprite shows the name and the current value in the top
left corner of the stage, updated every frame, and hideVariable("score")
takes it away. A sprite's monitor says whose variable it is, for example "Cat:
lives".

The game can be paused, stepped and slowed down: Window.getInstance().pause(),
resume(), isPaused(), step() (one frame while paused) and
setGameSpeed(0.5) for half speed. run(), timers, gliding, animations and
timed speech bubbles all follow, so the whole game stands still or runs in slow
motion while the window keeps drawing; keys and clicks still arrive, so a key
can resume the game. Timer.millis() and getDeltaTime() count game time,
which is the real time as long as the game is neither paused nor sped up.

Clones work like Scratch's. clone() makes a clone of the same class (a clone
of a Cat is a Cat, with a copy of its variables), puts it on the stage right
behind the original and runs its new whenStartsAsClone(), "when I start as a
clone". deleteThisClone() removes a clone and leaves the original alone, and
isClone() tells them apart.

public void whenStartsAsClone() {
  this.goToRandomPosition();
}

Before, clone() returned a plain Sprite that was not on the stage, so it
neither showed up nor ran the class's run(). A copy made with new Sprite(other) also drew with the other sprite's pen; it has its own now.