enable tooltip-label of Preference

browser/main/SideNav/SideNav.styl

@@ -75,6 +75,27 @@
     width 43px
     height 60px
     text-align center
+    &:hover .top-menu-label
+      transition opacity 0.15s
+      opacity 1
+  .top-menu-label
+    position fixed
+    display inline-block
+    height 30px
+    left 43px
+    padding 0 10px
+    margin-top -8px
+    opacity 0
+    margin-left 0
+    overflow hidden
+    background-color $ui-tooltip-backgroundColor
+    z-index 10
+    color white
+    line-height 30px
+    border-top-right-radius 2px
+    border-bottom-right-radius 2px
+    pointer-events none
+    font-size 12px
 
 body[data-theme="dark"]
   .root, .root--folded

browser/main/SideNav/index.js

@@ -150,6 +150,7 @@ class SideNav extends React.Component {
               onClick={(e) => this.handleMenuButtonClick(e)}
             >
               <i className='fa fa-wrench fa-fw' />
+              <span styleName='top-menu-label'>Preferences</span>
             </button>
           </div>
         </div>