Enhanced mobile `navigator` with routable hierarchy

[amcclain]

We would like to enhance the mobile navigator and NavigatorModel to support the common mobile navigation pattern below. (Note all names/terms are provisional and could be changed for clarity/consistency.)

• App defines top-level independent "stacks", most commonly switched between by a bottom-docked tab-bar style switcher.
• Each stack holds its own stack of pages, which can be drilled into and swiped back out of up to the top-level page within that stack.
• When reactivating an already-visited stack, it remembers the last page you were on and brings you back there, unless explicitly asked to rewind back to its top/default page.
• A new navigatorSwitcher component is optimized for showing 2-5 top-level buttons for stack navigation, expected to be shown at the bottom of the screen and always visible (excepting modal operations).
  • Tapping an already-active stack on the bottom switcher takes you to the top/default page in that stack, helping a user get back to where they started after a potentially deep drilldown.
• We'd establish a common pattern for a "More/Menu/..." stack that most apps would have.
  • This would take you to a page that could include an additional nested component for showing other independent stacks that were not quite important enough to win an always-visible place in the main switcher. We imagine these being a tiled list of icons that would visually relate to the bottom switcher (e.g. icon with centered text below).
  • That same page would commonly include common settings, about, logout buttons (or those could optionally be nested within a dedicated page underneath the ... stack, depending on the app/dev's preference.
• In future iterations, there could be user-optionality around which pages were shown on the top-level switcher vs a ... page menu, but we believe that would be possible after Improved, structured top-level app navigation #3931