New nav() api providing legacy support for shiny's tabPanel() logic

[cpsievert]

(To be merged after rstudio/htmltools#224)

This PR moves essentially all of the server-side logic behind {shiny}'s tabPanel() logic and exports it via a new API (in future PRs, we'll be expanding on this new API to provide new functionality). Here's a map from the old API to the new one:

Creating navs

• nav() (i.e., tabPanel())
• nav_menu() (i.e., navbarMenu())
• navs_tab() (i.e., tabsetPanel(type = "tabs"))
• navs_pill() (i.e., tabsetPanel(type = "pill"))
• navs_hidden() (i.e., tabsetPanel(type = "hidden"))
• navs_pill_list() (i.e., navlistPanel())
• navs_bar() (i.e., navbarPage())

Note that the navs_*() consumer functions (e.g., navs_tab()) generate a result that prints a static HTML page

tabs <- navs_tab(
  nav("a", "a"),
  nav("b", "b")
)
tabs

But you can also put them inside new page constructor functions that default theme to bs_theme() (the latest recommended version of Bootstrap).

page_fluid(tabs)

Updating navs

• nav_select() (i.e., updateTabsetPanel())
• nav_insert() (i.e., insertTab())
• nav_prepend() (i.e., prependTab())
• nav_append() (i.e., appendTab())
• nav_remove() (i.e., removeTab())
• nav_show() (i.e., showTab())
• nav_hide() (i.e., hideTab())