Adds AnimationContainer, which can be used to group a set of

app/slide_animation.h

Index: app/slide_animation.h
diff --git a/app/slide_animation.h b/app/slide_animation.h
index 15357fdc0bd253450f9416f7cd98c33573b81b97..3cf32d9276fd284b684e74b0ac0ed03ae062dd56 100644
--- a/app/slide_animation.h
+++ b/app/slide_animation.h
@@ -43,9 +43,6 @@
 // }
 class SlideAnimation : public Animation {
  public:
-  explicit SlideAnimation(AnimationDelegate* target);
-  virtual ~SlideAnimation();
-
   enum TweenType {
     NONE,          // Linear.
     EASE_OUT,      // Fast in, slow out (default).
@@ -55,6 +52,9 @@ class SlideAnimation : public Animation {
     EASE_OUT_SNAP, // Fast in, slow out, snap to final value.
   };
 
+  explicit SlideAnimation(AnimationDelegate* target);
+  virtual ~SlideAnimation();
+
   // Set the animation back to the 0 state.
   virtual void Reset();
   virtual void Reset(double value);