Closes #129: Trigger shown/hidden event for Shiny outputs in the sidebar

[bborgesr]

One thing I'm unhappy about is the jagged look of the expanded menu item when we first click it (so smooth transition, since the element actually has to be created)...

Example:

library(shiny)
library(shinydashboard)

ui <- dashboardPage(
  dashboardHeader(),
  dashboardSidebar(
    sidebarMenu(
      menuItem("Menu Item", tabName = "tab1", textOutput("text1")),
      menuItem("Menu Item", tabName = "tab2", textOutput("text2"))
    )
  ),
  dashboardBody()
)

server <- function(input, output) {
  output$text1 <- renderText("text1")
  output$text2 <- renderText("text2")
}

shinyApp(ui = ui, server = server)

[wch]

It would be a little safer to use .next('.treeview-menu').