The Extensions System should not depend on Chrome Apps.  This includes migrating all app-specific APIs out of the extensions system, as well as other app-specific concepts.  This makes ownership clearer, and prepares us for the ability to compile chrome with extensions enabled, but apps disabled.

Design Doc: https://docs.google.com/document/d/1KcJKXxYdFF9c29Uef_BClWZJPrGBjrhgYcOrLjeUxjo/edit#

(Internal only for now)