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

browser/main/SideNav/PreferenceButton.js

@@ -0,0 +1,19 @@
+import PropTypes from 'prop-types'
+import React from 'react'
+import CSSModules from 'browser/lib/CSSModules'
+import styles from './PreferenceButton.styl'
+
+const PreferenceButton = ({
+  onClick
+}) => (
+  <button styleName='top-menu-preference' onClick={(e) => onClick(e)}>
+    <img styleName='iconTag' src='../resources/icon/icon-setting.svg' />
+    <span styleName='tooltip'>Preferences</span>
+  </button>
+)
+
+PreferenceButton.propTypes = {
+  onClick: PropTypes.func.isRequired
+}
+
+export default CSSModules(PreferenceButton, styles)