Add cancel/close icon

src/images/icons/cancel.svg

+<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" viewBox="0 0 16 16" aria-hidden="true" focusable="false" class="Icon Icon--cancel"><g fill-rule="evenodd"><rect fill="none" stroke="none" x="0" y="0" width="16" height="16"></rect><path fill="none" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M8 8l3.536-3.536L8 8 4.464 4.464 8 8zm0 0l-3.536 3.536L8 8l3.536 3.536L8 8z"></path></g></svg>

src/sidebar/components/svg-icon.js

@@ -7,6 +7,7 @@ const propTypes = require('prop-types');
 // The list of supported icons
 const icons = {
   add: require('../../images/icons/add.svg'),
+  cancel: require('../../images/icons/cancel.svg'),
   'collapse-menu': require('../../images/icons/collapse-menu.svg'),
   'expand-menu': require('../../images/icons/expand-menu.svg'),
   copy: require('../../images/icons/copy.svg'),