Move sky-box,-button,-checkbox,-radio out of their directories

sky/framework/sky-button/sky-button.sky

-<!--
-// Copyright 2014 The Chromium Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
--->
-<import src="/sky/framework/sky-element/sky-element.sky" as="SkyElement" />
-
-<sky-element
-    name="sky-button"
-    attributes="highlight:boolean"
-    on-pointerdown="handlePointerDown"
-    on-pointerup="handlePointerUp"
-    on-pointercancel="handlePointerCancel">
-<template>
-  <style>
-    :host {
-      display: inline-flex;
-      border-radius: 4px;
-      justify-content: center;
-      align-items: center;
-      border: 1px solid blue;
-      -webkit-user-select: none;
-      margin: 5px;
-    }
-    :host([highlight=true]) {
-      background-color: orange;
-    }
-  </style>
-  <content />
-</template>
-<script>
-module.exports = class extends SkyElement {
-  created() {
-    super.created();
-
-    this.tabIndex = 0; // Make focusable.
-  }
-  handlePointerDown() {
-    this.highlight = true;
-  }
-  handlePointerUp() {
-    this.highlight = false;
-  }
-  handlePointerCancel() {
-    this.highlight = false;
-  }
-}.register();
-</script>
-</sky-element>