added tooltips like the new note button (and consequently extracted some buttons and styles)

browser/main/SideNav/ListButton.js

@@ -0,0 +1,24 @@
+import PropTypes from 'prop-types'
+import React from 'react'
+import CSSModules from 'browser/lib/CSSModules'
+import styles from './SwitchButton.styl'
+
+const ListButton = ({
+  onClick, isTagActive
+}) => (
+  <button styleName={isTagActive ? 'non-active-button' : 'active-button'} onClick={onClick}>
+    <img src={isTagActive
+        ? '../resources/icon/icon-list.svg'
+        : '../resources/icon/icon-list-active.svg'
+    }
+    />
+    <span styleName='tooltip'>Notes</span>
+  </button>
+)
+
+ListButton.propTypes = {
+  onClick: PropTypes.func.isRequired,
+  isTagActive: PropTypes.bool.isRequired
+}
+
+export default CSSModules(ListButton, styles)