Make it easier to include header/footer content in tabsetPanel()/navlistPanel()

[cpsievert]

navbarPage() has header/footer arguments for including content that should appear on every 'page'. This is a quite useful feature that is missing from tabsetPanel() and navlistPanel(). For example, if I wanted to add a link break bewteen the nav and content, I currently have to do that on every tab:

tabsetPanel(
  tabPanel("a", br(), "a"),
  tabPanel("b", br(), "b")
)

It'd be much easier to do:

tabsetPanel(
  header = br(),
  tabPanel("a",  "a"),
  tabPanel("b", "b")
)

This would also open the possibility of including inputs/outputs that should appear on every tab:

tabsetPanel(
  header = textInput("foo", "bar"),
  tabPanel("a",  "a"),
  tabPanel("b", "b")
)