add pause and resume methods to Animation

[shikaan]

I found myself reimplementing the following

  void pauseAnimation() {
    auto a = getAnimation();
    a.setSlowdownCount(STOP_ANIMATION_SLOWDOWN);
    setAnimation(a);
  }

  void resumeAnimation() {
    auto a = getAnimation();
    a.setSlowdownCount(a.getSprite()->getDuration());
    setAnimation(a);
  }

Over and over. It can legitimately be part of Animation

[shikaan]

We could also add something that helps with chaining animations/sprites.
Something like animation.setNextSprite("sprite", 90) which switches to "sprite" after 90 ticks.

Also I am not sure we should have frame duration as a property of the sprite, since it seems to belong to the animation. Moreover, non-native sprites (i.e., PNG sprite sheets) cannot have it

Reasons to keep having it in the sprite are:

• you may want to come back to the "original" frame duration instead of the one the animation is executing
• it's very handy to have it in the sprite editor
• it kinda goes well with the editorial flow: you make the sprite in the editor, you save the duration there, and the code is only responsible for loading it. Could be done by different people, in other words.