LibWeb: Stop spamming animation events on the wrong event target

This patch fixes two issues:

- Animation events that should go to the target element now do
  (some were previously being dispatched on the animation itself.)
- We update the "previous phase" and "previous iteration" fields of
  animation effects, so that we can actually detect phase changes.
  This means we stop thinking animations always just started,
  something that caused each animation to send 60 animationstart
  events every second (to the wrong target!)

Tests/LibWeb/Text/expected/WebAnimations/misc/animation-events-basic.txt

@@ -0,0 +1,2 @@
+    animationstart
+animationend