Root page

Fixes #53

Provides an explicit (instead of implicit) rootPage which can be any Page.

Providing "Drawer" is a rootPage gives the old behavior with a menu, where the menu "itself" is defined as an argument to Drawer.

Documentation about Drawer, version 6 of the app.json format, etc needs to be written/updated in /liger so when we pull this ticket in the info is available.