Add tabbed display mode and tab_strip member (#95)

This introduces the concept of "tabbed mode" for web applications, allowing web
applications to group multiple application contexts (tabs) in the same window.

* Adds a new display mode "tabbed" (for use with display_override).
* Adds a new member "tab_strip", to provide configuration for apps using this
  mode.
  * The manifest can configure a "home tab", creating a special tab that's
    always present, which will always show a page from a defined subset of URLs.
  * The manifest can configure a "new tab button" which opens a new tab at a
    specified URL.

The text of this change is based on an earlier draft by @loubrett (Louise Brett).