Add support for animationend event

This is triggered when an animation finishes. This is a high priority
because it allows us to start rooting nodes with animations in the
script thread.

This doesn't yet cause a lot of tests to pass because they rely on the
existence of `Document.getAnimations()` and the presence of
`animationstart` and animationiteration` events.

components/script/dom/mod.rs

@@ -213,6 +213,7 @@ pub mod abstractworker;
 pub mod abstractworkerglobalscope;
 pub mod activation;
 pub mod analysernode;
+pub mod animationevent;
 pub mod attr;
 pub mod audiobuffer;
 pub mod audiobuffersourcenode;