Organize sky/framework/animation

This CL cleans up the sky/framework/animation as follows:

1) I've moved code that's used only by the custom elements framework into
   sky/framework/elements/animation. This code is based on AnimationDelegates
   rather than Streams.
2) Rename ScrollCurve to ScrollBehavior because it encapsulates more behavior
   than just a curve.
3) Make the Generator interface explicit and mark subclasses as actual
   subclasses.
4) Move Simulation into generators.dart because it implements the Generator
   interface.
5) Move Animation out of generators.dart because it does not implement the
   Generator interface.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/1001373002

examples/stocks-fn/stocklist.dart

@@ -8,7 +8,7 @@ class Stocklist extends FixedHeightScrollable {
     Object key,
     this.stocks,
     this.query
-  }) : super(key: key, scrollCurve: new OverscrollCurve());
+  }) : super(key: key, scrollBehavior: new OverscrollBehavior());
 
   List<Node> buildItems(int start, int count) {
     return stocks

examples/stocks-fn/stocksapp.dart

 library stocksapp;
 
 import '../../framework/fn.dart';
-import '../../framework/animation/scroll_curve.dart';
+import '../../framework/animation/scroll_behavior.dart';
 import '../../framework/components/drawer.dart';
 import '../../framework/components/drawer_header.dart';
 import '../../framework/components/fixed_height_scrollable.dart';