add tooltip

browser/main/modals/PreferencesModal/StorageItem.js

@@ -313,16 +313,25 @@ class StorageItem extends React.Component {
               onClick={(e) => this.handleNewFolderButtonClick(e)}
             >
               <i className='fa fa-plus'/>
+              <span styleName='header-control-button-tooltip'
+                style={{left: -20}}
+              >Add Folder</span>
             </button>
             <button styleName='header-control-button'
               onClick={(e) => this.handleExternalButtonClick(e)}
             >
               <i className='fa fa-external-link'/>
+              <span styleName='header-control-button-tooltip'
+                style={{left: -50}}
+              >Open Storage folder</span>
             </button>
             <button styleName='header-control-button'
               onClick={(e) => this.handleUnlinkButtonClick(e)}
             >
               <i className='fa fa-unlink'/>
+              <span styleName='header-control-button-tooltip'
+                style={{left: -10}}
+              >Unlink</span>
             </button>
           </div>
         </div>

browser/main/modals/PreferencesModal/StorageItem.styl

@@ -47,8 +47,21 @@
   border-radius 2px
   border $ui-border
   margin-right 5px
+  position relative
   &:last-child
     margin-right 0
+  &:hover
+    .header-control-button-tooltip
+      opacity 1
+
+.header-control-button-tooltip
+  tooltip()
+  position absolute
+  opacity 0
+  padding 5px
+  top 25px
+  z-index 10
+  white-space nowrap
 
 .folderList-item
   height 35px