Devtools Animations: Add buffer and effect selection to animation timeline

Source/devtools/front_end/animation/module.json

 {
     "extensions": [
         {
             "type": "@WebInspector.ToolbarItem.Provider",
             "className": "WebInspector.AnimationControlPane.ButtonProvider",
             "order": 2,
             "location": "styles-sidebarpane-toolbar"
         }
     ],
     "dependencies": [
         "elements"
     ],
     "scripts": [
         "AnimationModel.js",
+        "AnimationGroupPreviewUI.js",
+        "AnimationUI.js",
         "AnimationTimeline.js",
         "AnimationControlPane.js"
     ],
     "resources": [
         "animationTimeline.css"
     ]
 }