title: shinydashboard header dropdown add group links
body: <p>I want to put multiple links within a dropdown menu in the header panel, but now I can only create it with a flat horizonal layout through tags$li, while I want a vertical grouped dropdown menu.  </p>
  
  <p>A minimal repeatable code is as below, I means I want to put the linkA and linkB under grouplinkAB, and users can open one of them in a new window. It may be achieved with dropdownMenu(type='notifications',...) as in the code, but I do not know where to put the group name of "grouplinkAB", and which can not open a new window when clicking on the link, also I have to hide the text "You have 2 notifications", so I want to achieve it with  tags$li and tags$ul, but I have little knowledge on HTML, any help will be appreciated.</p>
  
  <pre><code>library(shinydashboard)
  library(shiny)
  
  runApp(
    shinyApp(
      ui = shinyUI(
        dashboardPage(
          dashboardHeader(title='Reporting Dashboard',
                          tags$li(class="dropdown",tags$a("grouplinkAB",href="http://stackoverflow.com/", target="_blank")),
                          tags$li(class="dropdown",tags$a("linkA",href="http://stackoverflow.com/", target="_blank")),
                          tags$li(class="dropdown",tags$a("linkB",href="http://stackoverflow.com/", target="_blank")),
                          dropdownMenu(type='notifications',
                                       notificationItem(text='linkA',href="http://stackoverflow.com/"),
                                       notificationItem(text='linkB',href="http://stackoverflow.com/")
                                       )
          ),
          dashboardSidebar(),
          dashboardBody()
        )
      ), 
      server = function(input, output){}
    ), launch.browser = TRUE
  )
  </code></pre>